Sowter TVC LF Oscillation?
- Dave the bass
- Amstrad Tower of Power
- Posts: 12276
- Joined: Tue May 22, 2007 4:36 pm
- Location: NW Kent, Darn Sarf innit.
#1 Sowter TVC LF Oscillation?
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
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
"The fat bourgeois and his doppelganger"
#2 Re: Sowter TVC LF Oscillation?
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.
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.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
- Cressy Snr
- Amstrad Tower of Power
- Posts: 10581
- Joined: Wed May 30, 2007 12:25 am
- Location: South Yorks.
#3 Re: Sowter TVC LF Oscillation?
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.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.
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.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- izzy wizzy
- Old Hand
- Posts: 1496
- Joined: Fri Nov 02, 2007 7:02 pm
- Location: Auckland NZ
- Contact:
#4 Re: Sowter TVC LF Oscillation?
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
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
- Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
- Location: The Fens
- Contact:
#5 Re: Sowter TVC LF Oscillation?
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. (?)
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
- Dave the bass
- Amstrad Tower of Power
- Posts: 12276
- Joined: Tue May 22, 2007 4:36 pm
- Location: NW Kent, Darn Sarf innit.
#6 Re: Sowter TVC LF Oscillation?
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'.
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'.
"The fat bourgeois and his doppelganger"
- Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
- Location: The Fens
- Contact:
#7 Re: Sowter TVC LF Oscillation?
That's most likely the reason for it then!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.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
- Dave the bass
- Amstrad Tower of Power
- Posts: 12276
- Joined: Tue May 22, 2007 4:36 pm
- Location: NW Kent, Darn Sarf innit.
#8 Re: Sowter TVC LF Oscillation?
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.
The additional 5670 Buffer 'shields' the DAC op amp and Phono CF from the rufty tufty requirements of the TVC it appears.
"The fat bourgeois and his doppelganger"
- IslandPink
- Amstrad Tower of Power
- Posts: 10041
- Joined: Tue May 29, 2007 7:01 pm
- Location: Denbigh, N.Wales
#9 Re: Sowter TVC LF Oscillation?
"Rufty tufty" - much underused term - like it, respect due !
"Once you find out ... the Circumstances ; then you can go out"
- Dave the bass
- Amstrad Tower of Power
- Posts: 12276
- Joined: Tue May 22, 2007 4:36 pm
- Location: NW Kent, Darn Sarf innit.
#10 Re: Sowter TVC LF Oscillation?
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.
PS. I also look after the bags and get the crisps in too.
"The fat bourgeois and his doppelganger"
- Paul Barker
- Social Sevices have been notified
- Posts: 8987
- Joined: Mon May 21, 2007 9:42 pm
#11 Re: Sowter TVC LF Oscillation?
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.
He wanted to remain a reserved character. Quite shy.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
#12 Re: Sowter TVC LF Oscillation?
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)…..
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)…..
- Dave the bass
- Amstrad Tower of Power
- Posts: 12276
- Joined: Tue May 22, 2007 4:36 pm
- Location: NW Kent, Darn Sarf innit.
#13 Re: Sowter TVC LF Oscillation?
"Knobbled"!
I've just DL'd Audio-Tester BTW.
Onwards to measurement city.
I've just DL'd Audio-Tester BTW.
Onwards to measurement city.
"The fat bourgeois and his doppelganger"
- IslandPink
- Amstrad Tower of Power
- Posts: 10041
- Joined: Tue May 29, 2007 7:01 pm
- Location: Denbigh, N.Wales
#14 Re: Sowter TVC LF Oscillation?
Can you actually have a mini massif meet ?
"Once you find out ... the Circumstances ; then you can go out"
#15 Re: Sowter TVC LF Oscillation?
...got a nice ring to it, that has….IslandPink wrote:Can you actually have a mini massif meet ?