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TVC Preamps

Julf said:
I guess we have a different understanding of how a shunt attenuator is constructed. My understanding is that in a shunt attenuator the adjustable part is the shunt (connected between the output and ground).
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I was wrong (I think) about terminology, shouldn't have shunted...

But technically I think I'm OK.

The input impedance of a conventional passive is constant, of a 'shunt' it varies from the value of the series resistor to the maximum value of the shunt plus the series resistor.

The output impedance of a conventional passive varies from a minimum of 0 to a maximum of half the pot value. Of a shunt it goes from 0 to the series resistor in parallel with the maximum pot value.

I like the idea of a conventional pot at the input to the power amp, make it motorised or work it with a cheap R/C servo, and put the actual control in a useful place. Solves all passive problems. There may even be some minor hacking that could be done to the power amp input network that would be advantageous.

Paul
 
Paul R said:
I like the idea of a conventional pot at the input to the power amp, make it motorised or work it with a cheap R/C servo, and put the actual control in a useful place. Solves all passive problems. There may even be some minor hacking that could be done to the power amp input network that would be advantageous.

Paul
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Wouldn't it be simpler to buy a decent integrated amp and have done with it?
 
I have run TVC's since 2004. Owned two models both from Bent Audio in Canada using S&B transformers before S&B just stopped supplying him suddenly to launch MF. I had the chance to try various pre amps over the years but always stuck with my TVC until last year. I took a leap of faith and bought an ARC Ref 40 pre-amp and havent looked back since. The sonic improvement brought by the ARC was staggering, that was, until it blew up. So I am back with my trusty TVC while the ARC gets sorted.

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Tony, I used a Django tvc for quite a while. After removing all the junk and rewiring in silver it sounded exceptional but it just didn't have enough gain for my Jolidas so I went back to my valve preamp. In spite of the lack of gain I really miss it at times and I'm tempted to try a pair of Sowter 9395 transformers which offer both 6db and 12db of gain. Has anyone else any experience of Sowter attenuating transformers?
 
Chops54 said:
Tony, I used a Django tvc for quite a while. After removing all the junk and rewiring in silver it sounded exceptional but it just didn't have enough gain for my Jolidas so I went back to my valve preamp. In spite of the lack of gain I really miss it at times and I'm tempted to try a pair of Sowter 9395 transformers which offer both 6db and 12db of gain. Has anyone else any experience of Sowter attenuating transformers?
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I use Sowter 9335 transformers with OCC wire in a Glasshouse chassis. On their own they sound better than the standard Promitheus I kept for years, and as the volume control on my DHT pre sounds good to me.
 
I can't read electronics yet, but I'm interpreting that as the fixed 'shunt' resistor is in the signal (RCA centre) and the stepped attenuator in the signal return (RCA screen) - is that right?

I'm still struggling to understand the benefit or lack thereof of this approach assuming you are not cost-cutting. I guess it equates to a lower resistor count as you only need one 'side' of the stepped attenuator and a single resistor for the other side. I don't understand why it is inverted though, I'd have stuck the stepped attenuator on the signal rather than the return. I'll ask again; if one is doing this properly, i.e. spending £300 or more on a nice stepped attenuator switch populated with high-quality resistors (e.g. the one I linked to at HFC upthread) is there a gain to the shunt methodology?
 
The way some implement shunt attenuators is much about cost saving. Inexpenive pot coulped with the likes of a couple of £20 z foils. A full attenuator with z foils would be quite expensive though to my mind should sound even better.
 
Tony L said:
I can't read electronics yet, but I'm interpreting that as the fixed 'shunt' resistor is in the signal (RCA centre) and the stepped attenuator in the signal return (RCA screen) - is that right?
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Not really. There is nothing in the return path. An attenuator is a voltage divider. The full signal enters the attenuator, and then part of it is dropped over the series resistor, and then the rest is dropped over the shunt resistor on the way down to earth. That secod part is also fed to the output.

Are you familiar with Kirchhoff's circuit laws?

I'm still struggling to understand the benefit or lack thereof of this approach assuming you are not cost-cutting.
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It has nothing to do with price or component count. It is, as I wrote, about the misdirected belief that only components "in the signal path" affect sound quality.
 
Sorry, missed this. No, I'm not familiar with anyone's circuit laws I'm afraid. I have zero electronics training. I'm someone who can solder neatly and follow instructions, but that is my limit.

I'm still confused as to what direction to take, e.g. TVC, a better stepped attenuator, be it ladder or shunt. I just feel baffled by the options around what should be a very simple device. As such I'll try framing the question somewhat differently: I want the best possible 'preamp solution' for my Leak Stereo 20. This solution must add no noise, have suitable level adjustment to cope with the very high gain and be able to cope with things like tube phono stages upstream. The Leak has an input sensitivity of 125mV and a load of 1mOhm, I am not prepared to modify it in anyway. It is blameless in this, and that should be perfect for any passive preamp anyway. I have a feeling that a 100k stepped attenuator might be all I need if I think of this just as a preamp for the Leak, which I am prepared to do as this amp is a keeper for sure. Given the input load and sensitivity of the Leak is there any disadvantage to going for a 100k or even a 250k device? The more I think about it the more I suspect transformers may be unnecessary given this precise context.

I'm pretty sure for practicality I need a stepped attenuator as my DIY Tupperware passive (pictured on the previous page) is very loud by 'quarter to' (starting at 7 o'clock) whereas the Audio Synthesis has way more control ending up at about 11 o'clock/ten '3db' steps. I'm certain one of the posh 46 step attenuators would give me great control.

Budget up to £1k. I'm thinking of one of the better Glasshouse variants, say Seiden 46 way switch, top spec carbon resistors (apparently a bit warmer than metal film). Just need to nail down the best spec and establish that it will work with a nice tube phono stage should I want to use one (I actually have a Croft RIAA boxed up and rather like it aside from it is way bigger than it need be and I've not got anywhere to put it!).
 
Yes, a normal stepped attenuator of 100K is what I would recommend. 250K is getting a bit too close to the Leak input impedance, so you start getting interactions.
 
Ok, just to go back to the ladder vs. shunt thing again. I understand that a ladder is more of a constant impedance than a shunt, but I can't find any data as to how much. If I went for a 100k shunt how much should I expect the input and output impedance to vary, i.e. from roughly what to what? I ask as shunt seems way more popular with more choice of switches and resistors etc.

FWIW depending on system I seem to live between the -48db to -30db range on CD (the Audio Synthesis has front panel indication in db), so nowhere even close to wide open, nor that close to the bottom step (-66db).
 
Tony, if you are in the Manchester area (not sure what makes me think that) then I would be happy to bring my TVC over for a listen in about a months time. I get to hear your Tannoys and you the TVC :)
 
Ok, let me get my family moved down from Aberdeen to N.Wales mid-June. I will PM you once we are settled and we can go from there.
 
Tony L said:
Ok, just to go back to the ladder vs. shunt thing again. I understand that a ladder is more of a constant impedance than a shunt, but I can't find any data as to how much. If I went for a 100k shunt how much should I expect the input and output impedance to vary, i.e. from roughly what to what?
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Output: Zero to 100K in parallel with the series resistance (50K assuming a 100K series resistor)
Input: value of series resistor (assuming 100K) to value of series resistor + 100K, so assuming 200K

I ask as shunt seems way more popular with more choice of switches and resistors etc.
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Can't you just get a switch/variable resistor that allows either configuration, so you can try both?
 
Thanks for that. I'm assuming the extremes are at the extreme ends of travel, i.e. wide-open or closed? It looks like it shouldn't present a problem anyhow. I was just worried in case the input dropped low enough to upset a tube phono stage or whatever, the Leak should easily deal with the output given 1mOhm input impedance, i.e. 200k shouldn't be an issue.

Have a look at the HiFiCollective attenuator list here. I'm eyeing up the Seiden 46 switch and probably populated with the Amtrans carbon film, i.e. about as good as you can get and still fit inside a nice Glasshouse preamp case. I really like my rebuilt Leak Stereo 20, it is the best sounding amp I've ever owned, so I'm looking for the best possible preamp solution as I'll likely have it for the rest of my life.
 
I'd suggest you listen (eg mondie), borrow, or try second hand before committing to your "best possible preamp solution".

I've seen all sorts of discussions, particularly over on DIYaudio, which would suggest that carbon film can be the worst you can get.

As you're aware this is all in the ear of the beholder :) and could end up being an expensive experiment. At least if it's second hand you can pass it on at effectively​ no loss
 
Tony L said:
I'm assuming the extremes are at the extreme ends of travel, i.e. wide-open or closed?
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Yes.

Have a look at the HiFiCollective attenuator list here. I'm eyeing up the Seiden 46 switch and probably populated with the Amtrans carbon film, i.e. about as good as you can get and still fit inside a nice Glasshouse preamp case. I really like my rebuilt Leak Stereo 20, it is the best sounding amp I've ever owned, so I'm looking for the best possible preamp solution as I'll likely have it for the rest of my life.
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It looks pretty good.
 
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