Frequently Asked Questions


Do you do repairs and mods, or custom work to my vintage module?

Usually not, but maybe we can provide you with some info or recommend a tech and point you in the right direction.

Can I purchase direct?

No direct purchases at this time except for 1122 opamps and the LINE PAD-Z. Please contact these dealers and if there are any technical questions please contact us.

What do you think of lead-free solder? Do you use it?

We use lead-free solder and circuit boards as well as components that are RoHS compliant whenever possible. Sure, it’s not as shiny looking but it works well and lead-free solder is getting better with time and new technology. We recommend trying different brands and types to find the one that works for you. Old ways are difficult to change, but we must do the right thing.

Do you share schematics?

Sorry, but we don’t distribute, sell or trade any schematics as with any intellectual property and copyright.

What do you think I can do to learn the ins and outs of fixing old gear?

You learn by fixing old gear. Study some schematics, don’t ruin the circuit board with the iron or tools, be patient and be safe with electricity. Also, see if you can work at a recording studio that has a bunch of stuff that doesn’t work right. Use your creativity but stick to science, and never stop learning.


How can I run LINE level inputs into it from my PT’s or converters?

Because the MA5’s gain is stepped, it makes a very accurate and great-sounding line amplifier to run mixes through. It will add its signature harmonic overtones to the low end. However it is important to optimize the input load and output source impedances.

The LINE PAD-Z is designed for mic pre’s without line inputs, such as the MA5, enabling you to plug a line signal into the mic input.

The LINE PAD-Z adaptor serves three important purposes: 1) It provides the proper load to your PT or other converters without overloading them. 2) It drops the level -35dB, to the range of a microphone. 3) Its output impedance “looks” like a microphone to the MA5 preamp, so source impedances appear normal.

With the use of the LINE PAD-Z on the input of the MA5, unity gain would be at the 35dB position on the gain switch, with the output turned all the way up.

IMPORTANT: When coming out of the preamp into the A/D converter, you must go into the LINE input of your digital converter, not the MIC input which would add gain; otherwise you will have headroom issues and a poor signal-to-noise ratio. Some inputs are both, so you must switch to LINE input or use the TRS part of the combo XLR – read the manual to find out.

The LINE PAD-Z works very well and can be used with other high quality, low noise preamps. Not recommended for unbalanced interfaces. DO NOT ENGAGE PHANTOM POWER WHEN USING THIS LINE PAD!


Made with Switchcraft parts
Input impedance = 10K ohms
Attenuation = -36dB
Output impedance = 150 ohms

May be purchased with the MA5 preamplifier, or separately.

How much current does it draw?

75mA per rail, so 150mA total. This is about double compared to the current draw of the E27 since it is a class A amp with a big output transistor. But unlike class AB units, the current draw stays fairly constant, regardless of level or load to the output.

API 1608 info: It has come to our attention that the MA5’s initial current surge the first second of powering up causes the 1608 power supply to trigger its regulation and turn off. We will work together and hopefully come up with a solution that does not compromise the sound of the MA5, yet meets API’s console requirements. Please contact us with any questions.

It is equally important to work out how the power supply in a rack or lunchbox responds to the inrush of current to the modules in it. All power supplies we tested have current limiting regulation, meaning that the power supply rail will shut off if it senses a lot of sudden current draw, which can be triggered just by turning the rack/lunchbox on when full of modules.

So, always check to make sure all the lights are lit and everything is working when you first power up. If a rail does not power up, and the power supply had gone into its current limit mode, then turn it off. You will have to find a trim pot on the inside of the power supply which regulates the current limit and turn it up one o’clock at a time and retest each time until the power supply turns on without triggering the limit. Be absolutely sure it’s the current limit adjustment, and not the voltage adjustment, trim pot you are turning. Get a professional’s help to do this, or contact the manufacturer of the rack or lunchbox.

The BAE 11-space Rackmount comes with a large power supply. An example of what the trimpot looks inside is pictured here:


Please contact us if you have any questions and we’ll help you out.

What’s the 28kHz pushbutton for?

This was an idea borrowed from our E27 equalizer, but this time it’s only a 6dB rise and shelving rise at 28kHz. We realized it was so useful for bringing back life and realism to many instruments, we had to put it in the MA5.

It started when we first heard it on live orchestral music recorded by a friend- we could hear the sound stage and its reflections, resulting in a better depth of field. On acoustic guitars, it brought out the upper-end frequency timbre of the instrument. On drums, it sounds particularly nice on the room mic, capturing the natural high-frequency reflections better. Even dynamic microphones can be utilized in certain applications that you wouldn’t have used before.

For vocals, it brought the singer closer without turning up the sibilance frequencies. We thought it might be a good idea to put them on vocals, as many of today’s condenser microphones inherently have a bright top end, but not always in the right place for a particular voice. Adding 28kHz would allow you to use a de-esser or equalizer without losing too much of the top end detail, and without using too much additive eq later. This does depends on the characteristics of the microphone, so be careful not to put the 28kHz on everything!

There’s much more to the applications of the 28kHz feature… just experiment.

NEWLY UPDATED: About the DI....

After sound evaluations of three different DI circuits using different bass and guitars for this preamp, we found that we liked the Jensen direct box transformer the best – maybe because the sound went through the mic input transformer as well. It was an immediate winner in tests and had the fullness and definition in the low frequencies we liked.

Our best advice would be for you to purchase a high quality DI, like the one made by Jensen Transformers called the ISOMAX DB-2PX. It’s actually a great value because it is a 2-CHANNEL unit, and you could also use it in the way the Line Padz works, running a Line level signal throught it as well (-22dbu of level loss so it’s going to be a bit louder than the LPZ) You could have it mounted inside your rack and wire it up to your patchbay.

We sell it for $200 each directly. Call with any questions.

What’s the -6dB for on the output pot?

It’s a fine trim adjustment to get in between the 5 dB steps of the gain switch. It also mutes when turned down all the way, so you can exchange mics and prevent the speaker thump from phantom power.

No meter?

A meter would be nice for seeing if there is signal present, but considering how high the headroom is on this preamp (+31dBu into 10kohms, +27dBu into 600 ohms) the meter you should be looking at is the one for your recorder, like an A/D converter, which will clip at a lower level.

I’ve seen these with a Red gain knob. Can I get it that way?

Yes, just let your dealer know you want a red knob.

I see your MA5 being produced with a red or a black faceplate. What’s that all about? Just cosmetical reasons? Or is there any difference between the two?

There are no differences between faceplate or knob colors and it is purely cosmetic.

The first group of MA5’s had red faceplates due to a few dealer’s suggestions. I had always preffered a blackface (with the red highlights on the logo and name) so had them made but never showed it until a client asked if black was available, and it was. It has been the default color now.

There was also another color made for another dealer, a greyish color with green tint, which was a color matched from some old test equipment we had, but there are not many of these.

When installing them in my API rack, they’re a little tight due to the two screws at the top and bottom. What to do?

Place the lower part of the MA5 in first with the screws just inside, then just pull on the top slightly to get the top screws in. The lip on the API lunchbox or rack is bent a little too far in on some units, and using countersunk flathead screws on the MA5 means using weaker screws that can strip easily, instead of the stainless steel that used now. The original dimensions of the MA5 is correct.

Can I purchase output transformers from you for a DIY?

Sorry, no. The output transformer is custom-made for us through a research and design collaboration between Avedis Audio and Jensen Transformers, in a proprietary agreement. Furthermore, we remain even more vigilant with regards to the potential of the cheap copiers.

Seconds after powering down, it is normal to still hear a loud signal?

Yes, it’s the capacitors discharging but keeping the power up for a little while. This is normal for this particular type of Class A amp. It is recommended to mute your monitors or disconnect the preamps from your speakers or headphones when you power down.

Why do I hear a buzz/hum with the MA5 preamp but not with other pre’s I also have in the same rack?

The rack itself may have a grounding issue, either as a Pin 1 problem, a common (zero) not connected to chassis, safety ground not connected (unsafe and illegal), or some combination including all three.

The noise is really on all the preamps, but it will be more noticeable with the MA5 because the Class A amplifiers run with more gain than most other 500 series preamps, which have less gain from the op-amp but more step-up gain from the input transformer. Any noise that gets into the audio ground will be heard proportional to the gain, (for example, via building ground/conduit to pin 1).

Contact us if you’re having a problem with the rack; we will need to run tests and document any problems before having it fixed.

UPDATE: Please DO NOT use the MA5 or the E27 with an OSA rack. The problems with regulator inconsistencies causing high ripple noise, miswiring of the rack, and even safety is a risk to our modules.

2/19/10 – Please be careful not to pull on the output pot to remove the MA5 from the rack. This will damage the output pot and create a dead spot.


Why frequencies up to 28kHz?

When you boost or cut a chosen frequency, it is shaped like a bell unless you push in the shelving button – then it’s half a bell where the top of it extends beyond the selected frequency. The top of the bell would be 28kHz, for example, but the curve leading up to the top starts at frequencies within the fundamentals, or within 20khz. So you will easily hear the effect at the very tail end of the audible spectrum without accentuating siblance or already aggressive high frequency in the source. After you hear it, it will be part of your eq vocabulary.

Is it a clone or a reproduction of anything?

The E27 is not a clone or a reproduction of anything else. Certainly some things about it have been influenced by gear of the past and present, but the overall design is unlike any other.

Is there a characteristic sound to this EQ?

One way to tell is to have some music playing on your favorite monitors, and suddenly patch the eq in the chain, but without any boost or cut and even with the IN button not engaged. Listen carefully and what you will hear is the characteristic sound. Though the frequency response is flat when no eq is used, an increase in depth is often noticed with this simple test.

Nothing is transparent; everything has a sound.

Will your E27 equaliser plug into my rack?

The E27 should fit and work in all 500 series major brand racks, lunchboxes, consoles, and rackmounts like Brent Averill Enterprises 6 & 11 space rackmounts, and API Audio racks as part of the VPR Alliance. If there are any questions, please email us.

How narrow is the bandwidth?

The bandwidth (also called Q) varies depending on how much boost or cut there is. The more you boost, the narrower the Q, going from a fairly a broad bandwidth to medium.

Here is a graph of this Relative Q feature.

Why don’t the frequencies overlap?

Overlapping frequencies are popular with parametric eq’s which typically have a bandwidth control, but overlapping is not necessary for stepped frequency type of eq. When you overlap with a stepped eq, you force each band to have a wider range of frequencies than you really want. What if you want to adjust a siblance at 5K6 but still want to boost a little bit at 20Khz- if both of these frequencies are on the same band then you cannot use both. It’s much better for this type of eq to keep the bands separate but have the frequencies side by side.

It did not seem reasonable for this unit to putting 200 or 300hz in the midband and 1k2 in the low frequencies which may work for some things but limit more in other areas making it a heavy trade-off. The E27 does a great job of covering lots of areas between 33Hz – 28kHz

Can I also use it as an impedance converter? Or to interface unbalanced to balanced gear?

Yes. Because the input impedance of the E27 is 13k ohms and the output impedance is less than 45 ohms, this means that the E27 could interface two unlikely pieces of gear.

For example, say you had a CD/MD/DAT player that has unbalanced outputs and you wanted it to drive an old compressor or equalizer you have. The problem here is that the output of the CD player cannot handle a heavy load of 600 ohms and when you do connect them, you will likely find that you lose low end and the high frequencies don’t sound very good. You can connect an E27 in between them, and even with the eq button not engaged you still go through the transformers and the 1122 opamp, which now has enough output current to drive 600 ohms easily.

Another example is, say you have a console or outboard gear that has electronically balanced outputs and you need to interface it with something that is unbalanced, and let’s say that it’s that old compressor or equalizer again. Some electronically balanced outputs can handle being unbalanced if it has been implemented in the design, but some cannot deal with being unbalanced in any way. Even unequal impedances to ground in a balanced system, like passive attenuators using potentiometers can cause problems. If you try pluggin the two together, what you will hear is a loss of level as well as a loss of low frequencies because what you are doing is driving one of the push-pull opamps into ground – and it does not like that. Put an E27 in between the two, and due to the transformers on the input and output, everything is happily working together.

So what you are saying is that transformers are a good thing? What about those ads about "transformerless" circuits?

Transformers are invaluable for audio. They solve many real world problems. Well made transformers are inherently quiet (they are passive) and have a very high common-mode rejection ratio (CMRR- this is how well it rejects noise in a balanced line), they can be run unbalanced without needing any extra help, can handle high levels without severe clipping and they have a great range of characteristic sounds and applications. However, good quality transformer cost a lot more, they are heavy and take up lots of space in circuit boards, and not as readily available. These are not things you could easily clone overseas. Although there are very well made professional transformerless designs, some companies have decided to try and make “transformerless” a thing to desire – this is not a move for better sound but to cut cost in parts.

Much of the older or “vintage” quality gear which contain tubes are sought after because of their great sound but the key factor making those units sound so good isn’t much the tube as it is a fact that older designs have transformers.

But transformers are not made equal, and like many things, you can have good quality, poor quality and everything in between. A well-made transformer, designed and applied specifically for the job it will do, is itself the most valuable component even after many years of use.

Bell vs Shelf using the two push-buttons at the bottom... Explain.

The Low and High frequencies have a Shelf option which can be used to affect the band before the chosen frequency when using the Low band making it a great adjustable High-Pass Filter, or it could affect the band after the chosen frequency in the High band for larger scale tone shaping. See Graphs.

On the Low band, why does the Shelf start at 63Hz and not at 33Hz?

If you are at 33Hz, the -3db point with the bell is near 15Hz so it’s not really practical to have shelving at 33Hz with this eq. As the graph can also show, having a Shelf at 33Hz would not do any good.


Why does the 48V light stay on when I have EQ‘s in it and don’t need phantom power?

The light stays on to let you know that the power is available and working properly whether or not you will use it. The 48V phantom power is on pin 15, a pin that goes nowhere in an equalizer. Only preamps use that pin.

If the 48V does dim after you put in an eq, or preamp for that matter, then turn off the R52 and remove the module. Then turn the R52 back on and if the light is back on again, then something is not right with the module.

One exception known: There were a small number of 560A/AT graphic eq’s, mostly from the 80’s, which used pin 15 for external gain. It is a good idea to cut this pin on the circuit board to prevent damage to the unit. Please contact API or a qualified tech to do this for you or if you have any questions, please ask.

I‘m planning on using 550A’s for this rack and heard of the inputs being unbalanced. What should I be aware of?

Modules 550, 550A, 550b, 553, and 525’s all have un-balanced inputs (whereas the 550A-1, 554, and 560’s are electronically balanced input). Read more here. This is usually not a problem when interfaced with something which has a transformer output, like a 312A or 512C mic-preamp. But electronically balanced outputs don’t usually like driving unbalanced inputs, and you may get a 6 db loss and/or some other frequency response, phase response, or headroom problems.

The best way to remedy this is to place Line input transformer before each equalizer. A good transformer will interface with both balanced and unbalanced gear, capable of handling high levels, and maintain excellent common-mode rejection. Good transformers are not inexpensive, but it is the best solution for the best sound.

A good quality 1:1 transformer that behaves well with an 18.5k ohm secondary (the input impedance of 550A/b) should do fine. One option and recommendation would be made by Jensen™ transformers in their PI-2XX if you are looking for an external transformer unit.

Another option is to have us mount Jensen™ transformers inside the R52, which was designed to accommodate this if needed. The extra cost would be $200.00 for this option. Please mention this when ordering. But adding the transformer to the input will only make the R52 for equalizers that are unbalanced and you cannot use your preamps. Also, phantom power will have to be disconnected in case someone does put in a preamp and accidentally engages phantom power, sending 48VDC through the secondary of the transformer.

What does the LINK switch do?

It simply “splits” the signal coming out of channel one into two directions – channel one output, and to channel two input.

The LINK switch, sometimes called a “patch” or “thru” by the Australians, allows Channel 1’s output to connect to Channel 2’s input without cutting Channel 1’s output, so you could record the 1 and 2 outputs independently. This is great if you have a preamp on Channel 1 and an equaliser or compressor on Channel 2 and want to record both signals at the same time. Once the Link is engaged, Channel 2’s input is disconnected.

What do the three LED’s tell me?

The LED’s are there to tell you about power supply status. If there is a problem with a voltage issue with a module, one or more of the LED’s will be off. Remember that you must turn the unit off before installing a module. After you are sure it’s secure, turn it on and look for all three LED’s to be on.

The last green LED is for phantom power. Sometimes the tie-lines or the patchbay in the studio can have a short in the cables and phantom will not work although the green LED may still be illuminated. Be sure to check for proper continuity and/or resistance on the cables, tie-lines and patchbays before engaging phantom power.

How do I link up two 525 compressors for stereo?

We could add a toggle switch to the back which simply connects Pin 6 together for newer 525’s. Older 525’s will need to have Pin 7 connected together through a 121Ω resistor, but only if repinning correctly since the 525 originally had a different pinout than the equalizers. There is another option to run them in quad, using lower resistor values.

I have some old BAE racked 312’s. Will they work with this rack?

Because these preamps were built using older cards, the ones with the input transformer part AP2622 is too tall to be properly plugged into the R52 because the top of it prevents the connectors to meet. If you must only use those preamps, we can lower the connector by lowering the mating connector, but let us know… and remember that it’s not something you want to change every week because you cannot plugin other standard sized modules.

How do you fit a 512C mic-pre?

The 512C’s tranformer has some rather bulgey wires that need to be pushed down and squeezed into the R52’s slot. Usually just a little squeeze will do it, but if you need, you can also remove the top cover while fitting it into the slot, just remember to have the R52’s power unplugged.

The price is about the same as a lunchbox or 6 space rackmount. Why?

In comparing the materials and labor with the R52 to other 6 space racks, there’s not much more in the manufacturing process to get more channels. But the R52 is not a lunchbox and it fills a different need. And manufacturing quality gear cannot be made cheaply.

Why not manufacture it overseas?

That may lower the price to the end user and like all cheap imitations, there will be a compromise in quality. While many electronic components are made in other countries, there’s no reason why everything else can’t be made and assembled here in the USA. We chose US made parts and electronic components whenever possible.

Would you not pay a little extra if you were told the quality and serviceability would be better?

Can I link up two eq’s and use it as a 6 band?

Yes, good idea. You can do things like cut 200Hz and boost 63Hz, or cut 12kHz while boosting 24kHz.

Can I connect two mic-pre’s together?

Not a good idea.

What will happen if I have a preamp on the second channel and I have phantom on and then Link it to channel one’s output?

Looking at the previous question, you shouldn’t do it in the first place, but if you did, and the preceding module is transformer-balanced output, then phantom would see the DC resistance of the output transformer’s secondary and as long as there is no resistive difference in relation to ground between the balanced outputs of the transformer, nothing should happen and no current will flow. But don’t do it anyway because it serves no purpose.


Do you sell 1122 Opamps for DIY projects or personal use?

Unfortunately, we do not sell the 1122 opamp for DIY projects as we are no longer selling the 1122 opamp for any use other than for older API modules.