Don Audio CLASSI PEQ Questions

[Bonsaimaster]

Has anyone built the Don Audio Classi PEQ?

I was looking at this with the Gyraf tube make up gain.

I was looking at the board and there seems to be various versions with some modifications. Did Don Audio fix the boards to eliminate the need for these modifications?

The documentation with the boards does not have any values for the resistors and I see no schematic. Anyone know this values?

Can someone tell me about the bypass switch and how they achieved utility gain?

What other transformers than the Lundahl options. I assume 10K:10K on the input and 2400k:600 output.

Thanks in advance,

Bonsaimaster

 

[wrentema]

Hi.
some of your questions regarding resistors are being answered here on page 2.

https://groupdiy.com/threads/don-audio-classi-peq-vs-gyraf-g-pultec-g-pultec-cost.64844/
But I’m planning the exact same build (stereo Classi PEQ + G-Pultec make up). And it’s indeed weird that there’s no schematic and very little info..

 

[Bonsaimaster]

I received the 1.6V Classi Board from Don Audio. Important - this is different than other board versions and different than the one in the Don Audio manual. I ordered the capacitor kit for the this board however the kit was missing 39nf and 22nf capacitor and two 10K resistors. It did included a 1K resistor (for what I don’t know). There are two other alternative capacitors to change the frequencies of the eq settings (not used). After some back and forth and delays I finally got info from Don Audio that the resistors are:

R1 = 75R
R2 = blank
R3 = 10K
R4 = 10K
* These are also NOT indicated in the manual.

I ended up using the Don Audio toggle switch to put the unit in/out. I hooked it up to a two channel 12V relay I got on Amazon. The power came from the 9V tap on the G-pultec transformer running thru an Amazon kit rectifier giving me about 12.47V. This gives me a true bypass of the system and not just an in and out of the eq section. Apparently the EQ needs a pretty big gain and trying to bypass just the EQ section and keeping the make up gain stage in the loop was proving challenging.

For transformers I used an Ampex 10K:10K input transformer (I had one of these lying around) and a Carnhill 2290 output transformer at 2400:600 (2:1). I kept the Zoble resistors the same as stated for the Lundahl transformers in the G-Pultec BOM.

I ordered the missing capacitors from Mouser so waiting on them. Signal passes thru unit so far.

Another note make sure the IN OUT GND on the G-pultec make up gain and the Classi EQ board IN OUT GND are wired correctly.
G-Pultec IN (out of gain channel) ---> IN on Classi EQ
G-Pultec OUT (in of the gain channel) ---> OUT on Classi EQ
GND ---> GND

 

[timtom]

I wired up a Classi PEQ with the Gain Stage from Don Audio. The filter works correctly only as soon as I engage the filter the signal drops 20db.
The gainstage gives plenty of gain when I bridge the EQ section. Resistors are used like above.
Andy Ideas?

 

[TwentyTrees]

I wired up a Classi PEQ with the Gain Stage from Don Audio. The filter works correctly only as soon as I engage the filter the signal drops 20db.
The gainstage gives plenty of gain when I bridge the EQ section. Resistors are used like above.
Andy Ideas?

Hi, Andy here. Assuming that this is more or less a Pultec clone, the filter section would attenuate your signal by about 20dB. To maintain unity gain when switching the filter section in and out, include an equivalent pad in the bypass path - that way, the make up gain can remain constant whether the filter is switched in or out.

This block diagram from Ian Thompson-Bell's "Poor Man's Pultec" design shows you what I mean:



Do note that the Poor Man's Pultec filter section has a characteristic impedance of 10k, rather than the standard Pultec 600R. No idea which way Don Audio went, but if the pot values are the same as the standard Pultec then you might want to try eg 1k series, 100R trim pot and 47R shunt.

Alternatively (and probably more easily), do what Bonsaimaster did and rewire to bypass both the filter section and the make up amp.

Hope that helps!

 

[timtom]

I just hooked up only the eq with R1 75ohm R2 blank R3 R4 10k and I get 40db of loss.