FET-VALVE preamp (late-'93 model): compromised headphone amp circuit + other iss

KHashmi316 · « **on:** 22 Oct 2006, 05:55 pm »

Here are some images I snapped from the inside of this preamp during a routine contact-cleaning session:

For the first time since purchase, I inspected the circuit a bit more carefully and seem to have discovered a few potential compromises in design topology. Two notable compromises are in the headphone amp:

- First, see this Sept 1992 issue of Audio Basics: http://www.avahifi.com/root/audio_basics/ab_pdf/ab1992-09.pdf. On page 4, the schematic notes that there are "independent" or "dual" power supplies for each channel (I don't know what to call this scheme other than a "dual" power supply (correct me accordingly!). Specifically, note use of a *pair* of 2200uF/35V, 2200uF/35V and 1N4744A diodes *per channel*. If you look at the image of my preamp's headphone board, only a *single set* of the above components are used. Why was this done?

-Second, this April 1994 issue of Audio Basics http://www.avahifi.com/root/audio_basics/ab_pdf/ab1994-04-05-06.pdf outlines, among other things, new headphone-amp topologies (basic and buffered). Both use the "dual" power supply circuit, noted above; the buffered design is enhanced with dual LH0002CN's. This approach is putatively better (compared to the basic), but it was not used on my top-of-the-line FET-VALVE preamp built a few months before this issue went to print. I'll give AVA the benefit of the doubt here and chalk up the "compromise" to improvements made in the interim months.

More questions:

Are AVA preamps made after April 1994 offering a buffered headphone out circuit? And/or how has the circuit been improved. Indeed, AVA offers no *substantive* technical info on their products since the discontinuation of Audio Basics, years back.

I purchased my FET-VALVE preamp from AVA in late 1993. I also own an Omega III amp and have owned several other pieces of AVA equipment and kit products since the mid 1980s. Other issues with the FET-VALVE preamp include channel dropouts from poor-quality selector switches (rocker type), a flickering LED light, channel balance that changes based on volume control, etc. All this happened less than year after the unit was purchased. Not sure about whether these things would've been covered by the "warranty" but I elected to repair them myself because of incurred lead time.

I guess I'm a bit discouraged by the findings above since this was the top-of-the-line preamp from AVA at the time. Hopefully, FVA can reign in on why at least the power-supply "compromise" was made on the headphone board.

Other questions/issues:

While the AD843 used in the headphone circuit is a nice opamp, it is not as nice-sounding as the Burr-Brown (now part of TI) OPA627 (1). The OPA627 costs 2x as much as the AD843 but, IMO, that's what should be used in top-of-the-line units. What are AVA Transcendence preamps using now? In addition, how has the headphone circuit evolved since then?

(1) http://www.tangentsoft.net/audio/opamps.html

martyo · « **Reply #1 on:** 22 Oct 2006, 08:44 pm »

I have a Super pas 3i from '95 and your post got me curious. My unit does have the "dual" supplies. I have a question for you, I just replaced my pre-amp of 17 years with the 3i and my power amp is 14 years old. You mentioned a "routine" cleaning of the contacts. In my almost forty years of "hi fi-ing" I've never cleaned the contact on anything. Am I neglecting something, or does it have something to do with the environment. thanks

marty

KHashmi316 · « **Reply #2 on:** 22 Oct 2006, 09:34 pm »

Quote from: martyo on 22 Oct 2006, 08:44 pm

I have a Super pas 3i from '95 and your post got me curious. My unit does have the "dual" supplies. I have a question for you, I just replaced my pre-amp of 17 years with the 3i and my power amp is 14 years old. You mentioned a "routine" cleaning of the contacts. In my almost forty years of "hi fi-ing" I've never cleaned the contact on anything. Am I neglecting something, or does it have something to do with the environment.

Thx for checking.

The fact that your '95 Super-PAS has the dual supplies is good for you -- but sucks for me! (Take-home message for customers: know excatly what you're getting for your hard-earned $ -- hi-fi equipment isn't cheap.)

As far as cleaning goes ... I have used Caig products for years (AVA and other audio manufs also praise these products):

CAIG DeoxIT D5 Spray Contact Cleaner & Rejuvenator 5 oz.

CAIG DeoxIT Gold G5 Spray Contact Conditioner 5 oz.

A great kit here to start you off:

CAIG Audio Survival Kit

Kit contains: DeoxIT fast-acting deoxidizing solution that cleans, preserves, lubricates, and improves conductivity on metal connectors and contacts in 2 forms D5MS-15 mini-spray 5% solution, 200 ml, and D100L-2C squeeze tube 100% solution, 2 ml.; ProGold preservative and lubricant for plated metal electrical connections in 2 forms G5MS-15 mini spray 5% solution, 20 ml, and G100L-2C squeeze tube 100% solution, 2 ml; CaiLube precision lubricant specifically formulated to improve conductivity and lubricate conductive plastic and carbon compound faders, switches and other similar components in 2 forms MCL5MS-H15 mini spray 5% solution, 25 ml, and MCL100L-H2C squeeze tube 100% solution, 2 ml; plus lint-free swabs, brushes, and cloths.

Hopefully, AVA has moved to hermetically-sealed switches and pots. Their Dyna-based input-selector-switch designs -- as well as some of their highly-praised Noble pots -- are not ideal based on my experience.

mark funk · « **Reply #3 on:** 23 Oct 2006, 01:04 am »

I have a T5SLR built in late 2004 and it uses two AD843 with dual supply, I do not have headphones so I could not say how it sounds.

avahifi · « **Reply #4 on:** 25 Oct 2006, 04:32 pm »

Actually the headphone amplifier does use a plus and minus regulated power supply. The plus supply is simply not on the headphone board in this very old unit, is is derived from the regulated plus 12V supply for the tube heaters.

Not to worry.

The headphone amp has gone through three basic changes over the past few years, first a high speed op amp set for unity gain, later that combined with a hybrid current amplifier for higher current drive output, and in current production, simple very high speed, very linear, hybrid current amplifier chips alone. The headphone current buffer section follows the line stage and provides a low output impedance and high current drive for headphone applications.

Frank Van Alstine

avahifi · « **Reply #5 on:** 25 Oct 2006, 04:36 pm »

I could also note that all new production AVA preamplifiers use gold contact pushbutton switches now so the old switch dropout issue (that could occur if the tape-input switch was never used) is a thing of the past.

Frank Van Alstine

rlee8394 · « **Reply #6 on:** 25 Oct 2006, 07:20 pm »

Frank,
My '93 Fet-Valve preamp has the channel drop outs as well. I bypassed the tape monitor switch. Are these switches available? I'd like to replace them if possible. I may be trading in or selling it on eBay in anticipation of purchasing a T8 or Ultra Preamp.
Thanks,
Ron

KHashmi316 · « **Reply #7 on:** 25 Oct 2006, 11:20 pm »

Quote from: avahifi on 25 Oct 2006, 04:32 pm

Actually the headphone amplifier does use a plus and minus regulated power supply. The plus supply is simply not on the headphone board in this very old unit, is is derived from the regulated plus 12V supply for the tube heaters.

As noted in the orig. message (see above), "this Sept 1992 issue of Audio Basics: http://www.avahifi.com/root/audio_basics/ab_pdf/ab1992-09.pdf [clearly shows the better topology existed prior to the purchase of this preamp]. On page 4, the schematic notes that there are 'independent' or 'dual' power supplies for each channel".

Since I purchased the preamp (ordered new from AVA) in late 1993, I was puzzled by I was sent a compromised design, given that a better topology -- one with extra filtering caps and diode -- existed at least a year prev. And especially for a FET-VALVE unit?

For folks who need a little clarification, the image of the headphone board above is shown again here...

...the red arrow points to a jumper that is placed in place of where the additional rectifying components (caps and a 1N4744A diode) would've gone. The blue arrow shows the +12V supply being stolen from the tube heaters.

Quote from: avahifi on 25 Oct 2006, 04:32 pm

Not to worry.

The headphone amp has gone through three basic changes over the past few years, first a high speed op amp set for unity gain, later that combined with a hybrid current amplifier for higher current drive output, and in current production, simple very high speed, very linear, hybrid current amplifier chips alone. The headphone current buffer section follows the line stage and provides a low output impedance and high current drive for headphone applications.

Can you (or anyone else owning the a later model, e.g. Super-Pas, T7, etc) comment on what chips are hence (or now) being used?

KHashmi316 · « **Reply #8 on:** 25 Oct 2006, 11:55 pm »

Quote from: rlee8394 on 25 Oct 2006, 07:20 pm

Frank,
My '93 Fet-Valve preamp has the channel drop outs as well. I bypassed the tape monitor switch. Are these switches available? I'd like to replace them if possible. I may be trading in or selling it on eBay in anticipation of purchasing a T8 or Ultra Preamp.
Thanks,
Ron

I *think* those are made by CW:

http://www.cwind.com/switches.htm

You may have to dig for them using their internal search engine.

IAC, me thinks there may have been QC issue with these switches -- which are el cheapo and unrugged to begin with -- back during the early 90's or so. The dropouts I've experienced seem to be more mechanical (than chemical/corrosive) in nature. As far as gold contacts go ... I have plenty of non-gold switches on cheap receivers and walkmans that are holding up since way before the '93 FET-VALVE preamp purchase.

avahifi · « **Reply #9 on:** 26 Oct 2006, 01:28 am »

We will supply replacement original CW switches for free, just supply your name and address, preamp model, and original owner's name.

The switches work just fine if used so they can wipe and self clean. The problem is oxide buildup when they are never used at all.

Frank Van Alstine

rlee8394 · « **Reply #10 on:** 26 Oct 2006, 02:23 am »

Quote

As noted in the orig. message (see above), "this Sept 1992 issue of Audio Basics: http://www.avahifi.com/root/audio_basics/ab_pdf/ab1992-09.pdf [clearly shows the better topology existed prior to the purchase of this preamp]. On page 4, the schematic notes that there are 'independent' or 'dual' power supplies for each channel".

I believe that you are looking at a schematic for a stand-alone Versa-Kit board. That board had it's own independant power transformer. In that case, the extra caps, diode, etc. are required to generate the positive supply voltage. Since this board is installed in the Fet-Valve preamp which already had a regulated positive supply voltage, there was no reason to duplicate the effort. As Frank has mentioned, no need to worry. The headphone amp board and circuits have independant regulated supplies regardless of where they are generated. If you are still bothered by the fact that the positive supply is not generated from the board itself, you could lift the positive supply feed, remove the jumper, and install the missing components yourself. I doubt that you will notice any difference in sound though.

G E · « **Reply #11 on:** 26 Oct 2006, 02:52 am »

Quote from: KHashmi316 on 22 Oct 2006, 05:55 pm

While the AD843 used in the headphone circuit is a nice opamp, it is not as nice-sounding as the Burr-Brown (now part of TI) OPA627 (1). The OPA627 costs 2x as much as the AD843 but, IMO, that's what should be used in top-of-the-line units. What are AVA Transcendence preamps using now? In addition, how has the headphone circuit evolved since then?

(1) http://www.tangentsoft.net/audio/opamps.html

In the article you reference, Tangent states each opamp has its strengths and weaknesses, depending on the circuit and what one is trying to achieve, one may be a more appropriate choice than the other. Two OPA 627s are about $20 more than two AD843s (when Tangent wrote this article in 2002, revised 2004). Both devices are highly respected in the audio design and hobbyist community. If Mr. van Alstine had observed sonic benefit by using OPA 627's, he no doubt would have done so. I can't imagine $20 was a deal breaker.

G E

KHashmi316 · « **Reply #12 on:** 26 Oct 2006, 07:38 am »

Quote from: G E on 26 Oct 2006, 02:52 am

Quote from: KHashmi316 on 22 Oct 2006, 05:55 pm

While the AD843 used in the headphone circuit is a nice opamp, it is not as nice-sounding as the Burr-Brown (now part of TI) OPA627 (1). The OPA627 costs 2x as much as the AD843 but, IMO, that's what should be used in top-of-the-line units. What are AVA Transcendence preamps using now? In addition, how has the headphone circuit evolved since then?

(1) http://www.tangentsoft.net/audio/opamps.html

If Mr. van Alstine had observed sonic benefit by using OPA 627's, he no doubt would have done so.

Was an exhaustive analysis -- as tangentsoft.net has done in its survey -- done for the AVA preamp?

A developer doesn't have to go thru all of them. Certainly, there is enough literature and word-of-mouth advice to glean empirical evidence. But, at for least top-of-the-line gear like Trans. and FET-VALVE, some testing (and reporting of the test results in product literature) should be conducted/reported.

Tangentsoft's DIY designs are avail for free as his research -- an all-out effort which I truly respect.

avahifi · « **Reply #13 on:** 26 Oct 2006, 03:24 pm »

The gentleman complaining about the 1993 preamplifier shown above demands that I send him the latest headphone driver board, new switches, and a new volume control free of charge!

The unit is 13 years old and ten years out of warranty. The headphone amp does have a plus and minus supply and was the most current we did at the time.

I would appreciate any comments about how reasonable his demands are.

Thanks,

Frank Van Alstine

P.S. There is no more recent headphone amplifier circuit that will fit in the available real estate in this old chassis. I of course have offered another set of free rocker switches, but that is not enough I guess.

TjMV3 · « **Reply #14 on:** 26 Oct 2006, 03:43 pm »

Seems to me this gentleman is being ridiculously unreasonable. I believe we have seen what his true intentions are, in his approach and tone of his posts.

Interestingly, he's doesn't seem to be willing to sell-off his AVA Pre Amp. He still wants to keep it, even though he's trying desperately to attack and embarrass Frank.

Conversely, he is embarrassing himself.

Seems like very much .......shall we say, a duplicitous approach.

I think Frank is being more than reasonable and patient.

KHashmi316 · « **Reply #15 on:** 26 Oct 2006, 03:50 pm »

Quote from: avahifi on 26 Oct 2006, 03:24 pm

The gentleman complaining about the 1993 preamplifier shown above demands that I send him the latest headphone driver board, new switches, and a new volume control free of charge!

The unit is 13 years old and ten years out of warranty. The headphone amp does have a plus and minus supply and was the most current we did at the time.

I would appreciate any comments about how reasonable his demands are.

Thanks,

Frank Van Alstine

P.S. There is no more recent headphone amplifier circuit that will fit in the available real estate in this old chassis. I of course have offered another set of free rocker switches, but that is not enough I guess.

I *demanded* nothing. But, based on your switch-replacing gesture, yes, I did *request* the replacement of said items. Here is the substantive portion of the email I privately sent AVA:

Thx for your responses [to Audiocircle queries].

Unfortunately, I feel that the headphone amp issue is quite serious. The omission of the then-existing unity-gain/buffered and optimal-power-supply circuitry from your top-of-the-line preamp is regrettable.

In addition, there is the issue with the rocker switches and volume control that went bad in less than a year.

The unit was never stored for longer than one month.

All these issues do not exactly endorse trust in AVA products.

To remedy this, I request the following be sent to me:

- the latest, stuffed headphone board that will fit my 1993 FET-VALVE preamp

- a replacement volume pot

- replacement rocker switches

Send to:

[snip]

Regards,
-KH

budyog · « **Reply #16 on:** 26 Oct 2006, 05:43 pm »

Does this khashmi guy realise it is 2006 and the unit was made in 1993!

That was 13 years ago!

Does he really expect Frank or any manufacturer to believe stuff like "The unit was never stored for longer than one month"

Weather he is telling the truth or not, it is 2006! This is ridiculous.
He says "I demand nothing!

If you demand nothing then why are you saying :

To remedy this, I request the following be sent to me:

- the latest, stuffed headphone board that will fit my 1993 FET-VALVE preamp

- a replacement volume pot

- replacement rocker switches

That seems like demands to me and a whole lot to ask 13 years later!

Go away!

Merle · « **Reply #17 on:** 26 Oct 2006, 06:33 pm »

Classic A hole customer. 13 years and AVA should update his preamp! You gotta love it. Hell Frank, why not just send him your latest and greatest, free of charge? It's the least you can do! Seriously, people like that are the reason anything dealing with the public sucks. I buy and sell A LOT of used gear and it's getting to the point where it's just not worth the hassle. I had one guy on Audiogon tell me he would buy my amp, if I personally offered him a 5 year warranty! Imagine taking your 13 year old car back to the dealer and demanding the latest improvements! Where do these guys come from?

nathanm · « **Reply #18 on:** 26 Oct 2006, 08:10 pm »

Although it would seem silly to me to except replacement parts on something that old I don't think the tone of the message was bossy or demanding.

In descending order of politeness:

1. Wonder "I think it would be nice if I could get replacement parts..."
2. Ask - "May I have some replacement parts?"
3. Request "Please send me replacement parts."
4. Demand - "You must send me replacement parts!"
5. Threaten "I will kill you unless you send me replacement parts!"
6. Homicide "Now that you're dead, I will be stealing these replacement parts!"

#3 was chosen. Seems reasonable, just as reasonable as Frank saying, "Sure, if you pay for them!".

TjMV3 · « **Reply #19 on:** 26 Oct 2006, 08:18 pm »

Quote from: nathanm on 26 Oct 2006, 08:10 pm

Although it would seem silly to me to except replacement parts on something that old I don't think the tone of the message was bossy or demanding.

3. Request "Please send me replacement parts."

#3 was chosen. Seems reasonable, just as reasonable as Frank saying, "Sure, if you pay for them!".

I think it's unreasonable, arrogant and childish to expect replacement parts to be available at all, for a thirteen year old component, and equally absurd to expect to get them......... free of charge. The "free of charge" part being the ultimate in absurdity.

Not to mention that he could have approached this beforehand, in a civil and mature manner, by placing one simple phone call to Frank; as opposed to coming on the forum here and attempting to insult and embarrass Frank. Which of course he did not succeed in doing. Only himself.