AT-PEQ3 anyone?

[mlloyd1]

the regulator is a 78L15 - 15v at 100mA max.

[Fasterdamnit]

Update: I found a spare 24vdc wall wart and decided to see if it would be an improvement. I replaced all 8, 15v zener diodes and the regulator with 24V ones. The caps I installed previously were at least 25vdc. Just started playing it but can't really turn it up til later tonight. Sounds quite good at moderate volume. Another ~$3 thrown at the entry level pre...

[mlloyd1]

sounds likes a good idea.
several key op amps' specs (distortion, open loop gain, etc) improve with higher rails, at least up to a point.

mlloyd1

[h_a]

I had another short look wether that circuit takes also higher rails and indeed it seems as long as the caps and the opamps are suitable.

the japanese bipolars are only used as 2nd regulating stage after the IC-regs.

However, that was completely banana. Q1/Q2 act simply as 2 switches for whatever reason. These bjts run fully turned on during normal operation.

Hannes

[Fasterdamnit]

Originally Posted by h_a

I had another short look wether that circuit takes also higher rails and indeed it seems as long as the caps and the opamps are suitable.



However, that was completely banana. Q1/Q2 act simply as 2 switches for whatever reason. These bjts run fully turned on during normal operation.

Hannes

I made sure before I started soldering. I wondered why the transistors were there, current regulation?

[redrider]

I have recently finished upgrading my AT-PEQ3.

First of all I made a change to the power supply, which had to be replaced with a 230V one, so I built my own. Finally it was modified to be slow-turn-on in order to limit the thump when powering on the AT-PEQ, which could potentially kill a USB-powered sound card.

Second of all the original input capacitor was 1000µF 25V, the voltage rating of which is easily exceeded using the supplied 18V unregulated power supply, or any other suitable power supply. Therefore I replaced this one with a 35V type.

Furthermore I replaced the following capacitors:
Ceramic capacitors were replaced with PP film capacitors, except two 100nF bypassing caps.
The two 1.5nF Mylar capacitors were replaced with WIMA polypropylene
The two 5.6nF Mylar capacitors were replaced with Nichicon polyester (due to unavailbility of this value with WIMA and space restrictions)
The 1000uF 25V input capacitor was replaced with a 560uF 35V Panasonic FM (I would suggest to use acapacitor having no more than 20mm height, such as FM 470uF 35V, as this one was hard to fit.)
The 220uF 25V supply capacitors were replaced with 220uF 25V Panasonic NHG
The 10uF 25V bypass capacitors were replaced with 10uF 50V ELNA Cerafine.
The 47uF 25V feedback capacitors were replaced by 47uF 25V ELNA Cerafine.
The electrolytic coupling capacitors (0.47uF and 4.7uF) were replaced by WIMA MKS. By the way these capacitors are ideal for replacing electrolytics because they have the same lead spacing (2.5mm up to 1uF and 5mm up to 4.7uF). I prefer this in order to keep the leads short.

At this point I did not replace the NE5534, but if I wanted to then I would use the LME47920 (PIN compatible DIP version). I have used the LM47910 as a replacement for the NE5532 in another project.

I have observed two deviations from the schematic:
1. My unit is using a 78L18 regulator, not a 78L15 one.
2. The 100nF ceramic capacitor is across the input and output pins of the regulator, not across input and ground (although I do not understand the purpose.)

In case someone wants to perform th esame upgrade, all components can be sourced from Banzai Music. The AT-PEQ3 can still be sourced from LP Gear, although the price has increased since I purchased my unit.

I did not have the time to compare the sound before and after the modifications, but wanted to make some basic improvements. Soundwise I am very happy with the unit, and I must say that its mechanical construction is very solid.

[redrider]

Correction: the AT-PEQ uses a NE5532. The replacements are:

NE5534 --> LME49710
NE5532 --> LME49720

[kevinkr]

Originally Posted by Fasterdamnit

I made sure before I started soldering. I wondered why the transistors were there, current regulation?

They're there for pop/thump mitigation nothing more, they disconnect the supply from the op-amp when the voltage is less than approximately 7V..

Pay attention to the cartridge load which stock is only ~32K. Replacing the 47K input resistor with something like 90.9K will get you close to the recommended 47K (47.6K).

Dump the zeners on the inputs and outputs - they're there for nothing more than cheap ESD protection, and with just a little care should not be be necessary under most circumstances. A properly chosen TVS would be a better choice than the zeners on the supply, and a resettable fuse between power connector and TVS/pre-amp circuitry would provide protection against an external supply malfunction or backwards connection.

Higher supply voltages cannot hurt, but if running on 24V supply, you should change any 25V capacitor across the rails to 35V - any others should be OK as is.

[redrider]

Thank you so much for explainig the transistor circuit. I am positively surprised that this was given some thought, and it is probably effective when unplugging the power supply. However it should not change much (if at all) during power-on.

[kevinkr]

Originally Posted by redrider

Thank you so much for explainig the transistor circuit. I am positively surprised that this was given some thought, and it is probably effective when unplugging the power supply. However it should not change much (if at all) during power-on.

Yeah, the half supply bias network has a long enough time constant I suspect to handle the turn on thump if not completely eliminate it. Obviously some thought went into power sequencing I imagine with a view to not generating speaker damaging transients if possible.

Overall this is a much better design than the ones that were around when I was growing up and made the transition from a ceramic to magnetic cartridge.