Apt Holman Preamplifier Tone Switch Pop

[goldswimmerb]

1. Power budget is rarely considered when "rolling" opamps.

I'm sure T.Holman designed the power supply to adequately cover the worst case draws from TL072s plus some reserve, but did not anticipate significantly more.

IMHO the OPA2134 is a poor choice in that regard, as it consumes nearly 3x more current than the original TL072, and may push the power supply to, or over, its capability.

Ranked in increasing power requirement (per amp, 2 amps per chip):

• TL072: 1.4mA nom, 2.5mA max
• TLE2072: 1.7mA nom, 2.2mA max
• OPA1642: 1.8mA nom., 2.3mA max
• OPA2134: 4mA nom., 5mA max

The OPA1642 is said to be an "improved" 2134, with some better specs, and significantly less power requirement. Unfortunately, it's SOIC only, so not as DIY friendly, but that's what I used and they work great with the original sealed transformer.

Another issue rarely considered is power pin bypass caps. The Apt has a number of them, "good enough" for the TL072, but lacking for the more "high strung" opamps. More should be added when "rolling".

2. The electrolytic coupling caps CAN be entirely replaced with poly, if one is willing to put in the time, effort & money.

Thanks for the insight, I will keep this in mind if I ever get another one of these, I want one for myself so I may have to go "all out" if I ever get one. Even this one, with all but 1 op-amp stock, new Transistors, new caps (mix of Wurth, WIMA and Nichicon) sounded stellar.

 

[mcgjohn]

@HalfApt I always replace the 072 with a 2072- did not mean to "mansplane" the circuit

@mcgjohn - The early versions had a sealed transformer which was tiny and does fail. If the unit has that transformer, I replace it with a Triad 700ma unit, which keeps things in the same footprint. There is also a torodial that will fit. I have considered an outboard supply, though keeping the original external looks has been important, as has the switched AC for the APT power amps on the rear. I believe the secondary is 18V.

I have wondered what a fully regulated supply using LM series regulators with a larger transformer and a substantial set of power supply caps would do.

Thanks for the added input. In the years we were working on them, don't recall seeing a trans failure, but not to say it could not happen. 700ma is still pretty small. For dynamics you need to get into the 4 to 5 amp range.
As others have mentioned you can keep the stock appearance if you outboard the transformer.

 

[GKTAUDIO]

700ma is still pretty small. For dynamics you need to get into the 4 to 5 amp range.

The preamp is rated as drawing a peak of 125mA. Can you please elaborate on how it could draw 4-5A during dynamics? It seems to me that at 36V (+-18V) and a rated current draw at the primary of 125mA, DC current draw would be in the 0.5A range? The preamps only have 1A Vregs and 1,000R main filter caps - how could this unit ever draw so much current?

In the years we were working on them, don't recall seeing a trans failure, but not to say it could not happen.

One of the units I have had a transformer that failed shortly after it was initially sold by Apt. It had no modifications, and was like new, as it hadn't been used in 30 years. The second unit failed after I swapped op amps, and the current draw clearly exceeded the capability of the later style transformer. I've heard of others, including that reported here by pwauk12.

As others have mentioned you can keep the stock appearance if you outboard the transformer.

Yes - one way would be to replace the 18V out RCA jack with a 3-pin power jack - that would be very close to looking like stock. Not sure anyone would have a use for the 18V out anymore.

 

[mcgjohn]

The preamp is rated as drawing a peak of 125mA. Can you please elaborate on how it could draw 4-5A during dynamics? It seems to me that at 36V (+-18V) and a rated current draw at the primary of 125mA, DC current draw would be in the 0.5A range? The preamps only have 1A Vregs and 1,000R main filter caps - how could this unit ever draw so much current?



One of the units I have had a transformer that failed shortly after it was initially sold by Apt. It had no modifications, and was like new, as it hadn't been used in 30 years. The second unit failed after I swapped op amps, and the current draw clearly exceeded the capability of the later style transformer. I've heard of others, including that reported here by pwauk12.



Yes - one way would be to replace the 18V out RCA jack with a 3-pin power jack - that would be very close to looking like stock. Not sure anyone would have a use for the 18V out anymore.

Agree, not much use for an 18V supply anymore. Since there is limited use for that supply we would remove it and run the secondaries in through that whole after it was enlarged a bit.

the unit will likely not draw that much current, but will have a huge supply to pull peaks from. Have done the same on the Adcom GFP series, Yamaha CX series, Halfer preamps and other preamps. The results have always been the same, big dynamics more of an ease to the sound and improved sound stage.

Wish I could claim credit for the idea but can't. Paul McGowen at PS Audio tried it with the PS Audio 5.0 preamp. Offered it with a standard transformer and as an option, offered the same model with an outboard 500 watt transformer.

We got one in the shop for a minor repair. After the repair we gave it a listen and could not believe how much better the preamp sounded with the larger trans.