Insert EQ preamp fixed 30-40db gain

[maosante]

Hey guys,

Recently I purchased the Don Audio PEQ which is a passive EQ but I didn't bought their gain stage circuit because it is opamp based, and the whole purpose of this EQ is to be a fully analog device with no ICs. The Lundahl gain staging they offer is way to expensive for me. So I have this idea of using the EQ as an insert of my loved GAP Pre-73 mk3, yet when I connect it I loose about 40db, which gives me no alternative to gain up my preamp leaving lots of headroom out and making the sound a bit squashed, tight and much more distorted, this sometimes is a desired sound but it would be nice to be a choice and not a condition.

My idea to add this 30-40db is by using the stock input transformer of the GAP, leaving distortion and noise out of the equation. The insert is unbalanced and after the EQ it will go through the output transformer and voilá, clean sound with great headroom will be out.

In order to achieve this I would need a power supply to feed the input trafo, and a few configurations to add those 40db. I've search if there is a simple power supply and fixed gain circuit for this purpose but had no success. Any ideas or PCBs out there you can share?

Here are some pics for inspiration... lol

 

[abbey road d enfer]

My idea to add this 30-40db is by using the stock input transformer of the GAP, leaving distortion and noise out of the equation.

You won't find a xfmr that provides 30dB gain. And remember, a xfmr is not magic, it may increase voltage, but will decrease the available current, which will make it unable to drive the following stages.

The insert is unbalanced and after the EQ it will go through the output transformer and voilá, clean sound with great headroom will be out.

Not clean and no headroom because of the impedance mismatch and power losses.

In order to achieve this I would need a power supply to feed the input trafo

Transformers are passive components, which don't need powering.

, and a few configurations to add those 40db.

Do you mean an active circuit? That is more reasonable.

I've search if there is a simple power supply and fixed gain circuit for this purpose but had no success. Any ideas or PCBs out there you can share?

There are many that you can easily configure for your application.

https://fr.aliexpress.com/item/1005...d=2023022210153310502827950400890002287549_10
https://fr.aliexpress.com/item/4000...2!sea!FR!747477913&curPageLogUid=VnnaJJkJovcj
But I don't know if they are available for you.

 

[maosante]

Hi, thanks for your straight answer.

I'm still learning all about this, the second pcb you send seems to be a good option, yet, I would like to use the xformer first before adding an active circuit, if it is possible (don't really know). You mention current losses, does this change the sound in any way? I wonder if there's a way I can configure the transformer in the picture to its maximum level within this insert unbalanced circuit?

Here you can check the xformer winding.

http://www.jlmaudio.com/CL7602/CL7602 mic input 3.jpg

I also have another output transformer. 2:1 configuration, the problem is that I don't know how to make the connections since the input and output share the same cold / ground signal.

Hope you guys can help me figure it out. I'll be experimenting by the way.

Thanks.
Mauricio.

 

[abbey road d enfer]

I'm still learning all about this, the second pcb you send seems to be a good option, yet, I would like to use the xformer first before adding an active circuit,

What for?

if it is possible (don't really know). You mention current losses, does this change the sound in any way?

Yes, and the levels are not correct.

I don't know how to make the connections since the input and output share the same cold / ground signal.

They share the same "ground", yes, but not the "cold".

 

[TwentyTrees]

Rather than trying to use your PEQ as an insert in your preamp, where as Abbey notes the impedances are all wrong, one relatively straightforward alternative is to configure the PEQ to use the preamp as a make-up gain stage after the filter.

From a quick review, it looks like the Don Audio PEQ is basically a Pultec filter-stage clone, with a characteristic impedance of 600R. All you need then is two transformers, a box, and some connectors. For the input, 600:600 is traditional on a Pultec; for the output, 600:150 (2:1) would give you an output impedance that looks like a microphone to a preamp. You can then plug the output of your PEQ into the input of whatever mic preamp you want to use for makeup gain - you'd need around 26dB on your preamp.

Alternatively, you could use a 10k:600 input transformer for a more modern bridging input impedance (in case you've anything that would struggle to drive a 600R load). You'd then need an additional 12dB makeup gain on your preamp.

 

[k brown]

Don't some of those old Altec octal plug (1588, etc). units that are gain block and transformer in a can do about 30dB gain?

 

[jokeramik]

You won't find a xfmr that provides 30dB gain. And remember, a xfmr is not magic, it may increase voltage, but will decrease the available current, which will make it unable to drive the following stages.

There are some technical problems:
1. after passive EQ you need a booster-amp with with a relatively high input resistance in order not to load the filter circuit.

2. The transformer square the resistance. So have a high level output with a high impedance. You should also know if the transformer requires a special termination network for best impulse response.

Best regards!