Sowter TVC LF Oscillation?

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Dave the bass
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#1 Sowter TVC LF Oscillation?

Post by Dave the bass »

Here's a funny one.

Yonks ago I purchased a 2nd hand pair of Sowter TVC (9335) and a 23 position Seiden switch. I finally got round to boxing them up to make a stand alone Passive pre, literally just the switch + 2 x 9335's + 4 Phono sockets.

Now then, fitting it in place of a resistive pot-in-a-box volume control I became aware of a very low frequency oscillation coming from the speakers, its also visible in the 4 ammeters on the bench supplies in that the current demand fluctuates (pulses) when the volume is turned full up. Odd.

It does it most when the system is 1st powered up from a cold start. It later appears damps its self, no really.

It only seems to do it on when Phono is selected, not the DAC (AK 4396). The phono is a WD Phono 3 , so CF out, a large cap 2.2uF blocks DC in that cct. Could it be an interaction betwixt 'C' of that cap and 'L' of the TVC creating the oscillation?

Is there a preferred way of 'driving' a TVC BTW? Many years ago our-Nick did a 2A3 pre with an active load that IIRC he took to Owston that drove a TVC rather than just blindly relying on the source being able to work into the TVC primary happily.

Have I done summat bad?

DTB
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Nick
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#2 Re: Sowter TVC LF Oscillation?

Post by Nick »

Try a 100R resistor in series with the CF output. You may be finding the C output cap and L of the TVC (as you suspect) is forming a tank that is oscillating. The R may help to damp it out. Its made worst by the bugger all current a ecc83 can provide as a cathode follower.

More C in the phono B+ may also help, I think it will be pulling the B+ down which the first stage sees, and then its in effect motorboating.
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#3 Re: Sowter TVC LF Oscillation?

Post by Cressy Snr »

Nick wrote: More C in the phono B+ may also help, I think it will be pulling the B+ down which the first stage sees, and then its in effect motorboating.
Yes, my old WAD Phono II used to do that. It was interesting watching my speaker cones moving back and forth at around 1Hz when the unit was idling.
It was around 10 years ago so I can't remember the cap value I put on the end of the B+. Might have been 470uF but I'm not sure so don't quote me on it Dave. :)
It might even have been you Nick that suggested the cap.

It did work though.
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#4 Re: Sowter TVC LF Oscillation?

Post by izzy wizzy »

Had the same thing. I had a 1u output coupling cap. Read about a thing called lossy parafeed from Voltsecond. It's called something else now.

Idea is stick a larger parallel cap in but it probably won't be the same quality as the existing cap. A 1k in series with the bigger cap seems to minimise its bad effects.

It's not something you can ever get rid of entirely (the CL resonance) but you can push it down to the point where it doesn't have gross effects.

TVCs are not "just stick it in" type devices. Most times they have to be 'fettled' a bit in circuit.

cheers,

Stephen
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#5 Re: Sowter TVC LF Oscillation?

Post by Mike H »

Dave might also try sticking a high value resistor between input and ground of the TVC, that should apply some damping to the inductor. Value depends what minimum input load do you want it to look like from the external source, but try say 100k, see what happens. (?)
 
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#6 Re: Sowter TVC LF Oscillation?

Post by Dave the bass »

Many thanks for the insight into the problem Gents, much appreciated. I've opted for an even more drastic 'fix' for now... I've installed an old CCS'd 5670 unity gain buffer I've had knocking about for ages betwixt sources and the TVC! Works a treat actually.

Nick,no probs, I can add more C to the B+ rail but after a quick look at the Phono 3 schematic I now realise its already got a 1M + 1K potential divider on the output so I'm guessing a 100R might not be enough to remove the motorboat excitement perhaps?

Izzy, I looked up Lossy Parafeed and I confess I glazed over, I much prefer your explanation of cap and resistor! I thought the TVC might not be a fit and forget drop in otherwise everyone'd have have one I suppose, bum! I was hoping for a simple little 'bung-it-in-a-box-project' whilst I had a day spare.

Mike, I'm a bit worried about adding a 100K in // with the sources, my reasoning being that I think they're struggling already at the moment driving the TVC direct. The phono coped better than the DAC output to my ears but both have an 'uneasy' feeling to their sound until I bunged the Buffer amp after the source selector but before the TVC.

I'm still a-ponderin'.
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#7 Re: Sowter TVC LF Oscillation?

Post by Mike H »

Dave the bass wrote: Mike, I'm a bit worried about adding a 100K in // with the sources, my reasoning being that I think they're struggling already at the moment driving the TVC direct.
That's most likely the reason for it then!
 
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#8 Re: Sowter TVC LF Oscillation?

Post by Dave the bass »

Yeah Mike, I think thats what Nick was pointing to also in that the ECC83 CF stage of the phono doesn't have much in the way of driving grunt.

The additional 5670 Buffer 'shields' the DAC op amp and Phono CF from the rufty tufty requirements of the TVC it appears.
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#9 Re: Sowter TVC LF Oscillation?

Post by IslandPink »

"Rufty tufty" - much underused term - like it, respect due !
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#10 Re: Sowter TVC LF Oscillation?

Post by Dave the bass »

Yeah, I just threw that phrase into the conversation when I was down the pub with Alan Blumlien and Morgan Jones the other day. Up to that point it was just me, Paul Voight and Nelson Pass chewing the cud but as soon as 'Al' and 'Morg' join in things the theory and descriptors get all a bit wafty. I reel 'em in and ground 'em.

:)

PS. I also look after the bags and get the crisps in too.
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#11 Re: Sowter TVC LF Oscillation?

Post by Paul Barker »

I was in a social with Morgan Jones, and at the time it wasn't public knowledge what his forum nom de plume was. I quoted his nom de plume in the conversation, and he said, yes I know, I said that!

He wanted to remain a reserved character. Quite shy.
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#12 Re: Sowter TVC LF Oscillation?

Post by thomas »

I had the privilege (?!) of hearing DTB's low freq. oscillation today when he brought his nicely boxed up WAD phono and Sowter TVC round mine for an impromptu select Kent Massif meet, and very impressive it was too! The buffer sorted it though…..

I promise to play some more Beefheart later DTB!

…and I hope I didn't mess up your phonostage too much (hehe, twirls evil moustache, etc)…..
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#13 Re: Sowter TVC LF Oscillation?

Post by Dave the bass »

"Knobbled"!

I've just DL'd Audio-Tester BTW.

Onwards to measurement city.
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#14 Re: Sowter TVC LF Oscillation?

Post by IslandPink »

Can you actually have a mini massif meet ?
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#15 Re: Sowter TVC LF Oscillation?

Post by thomas »

IslandPink wrote:Can you actually have a mini massif meet ?
...got a nice ring to it, that has…. :D
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