Transformer Volume Control

[ToolGuyFred]

Anyone tried a transformer volume control? For the uninitiated, this is a single winding transformer with a tapping for each volume setting, so an audio auto-transformer. I am currently running a Foreplay with Sweet Whispers but the switches on the stepped attenuator are now well past their best, so I am considering some sort of replacement. Obviously the Submissive would be suitable but I am also considering the Glass Audio transformer volume control. Although the Submissive is a lower price, by the time I have paid value added tax and import duty to the UK, there is little to choose between them on price...

 

[Tubejack]

I built a TVC using these SILK components  http://www.acoustic-dimension.com/transformer/silk-audio-23-step-tvc-main-page.htm

While I was at it, I included a passive (resistive) volume control that I was able to switch between the 2 types of volume controls.

My sources are digital DAC, tube vinyl pre (EROS & others), Tape (thru Tube Repro), bi-amped speakers thru 2A3 and 300B amps using active xover.  With my lower power amps and the passive volume control's insertion loss, I really needed more gain; and I don't think all the various source impedance's played well with the TVC - the "magic" just wasn't there, so I went back to a tube line-stages that give me 6-8db gain.

My conclusion was a passive volume control is very system dependent, including ICs used, etc.  YRMV.

 

[Paul Birkeland]

I agree that TVC's and AVC's are intensely system dependent.  I did see a nice pair of S&B transformer volume control transformers on Doc B's desk that have no purpose currently...

 

[ToolGuyFred]

OK, after much consideration, I've taken the plunge. (This is very much how things went with deciding to go Lowther: you have to try it in your system to hear if it works.)

I have gone for the mark 2 version of the standard Glasshouse TVC. I only have about 50 hours "on the clock" but it has already changed noticeably. Initial listening was inexplicable and slightly frustrating. I can hear more detail than with my Foreplay but individual performers were harder to pick out. This combined with a slight veiling which made me question my decision. Both of these features are improving with use and I expect it to take several hundred hours of use to stabilise properly - particularly as I have opted to use Teflon sleeving over solid silver wire - the standard option is soft copper with cotton insulation.

Will post again after more listening.

 

[Hank Murrow]

Yes, and this is how I did it. I bought a pair of I.A.G. TVC's which use Sowter transformers when I had Paul Birkeland build my Sister/Kugelis Linestage back in 2005. I auditioned it in Doc's big system at BH and did not find it lacking in any way. These TVC's have performed flawlessly ever since, giving me 27 steps from the Shallco switches which are quiet and high quality devices.

In the version Paul built, this Linestage uses EL84 current-sourced 6C45Pi tubes to drive a pair of MQ B7 Cobalt core output transformers with V-cap TF/TF caps. I include a couple of pics to show how PB implemented Doc's original circuit. The hybrid shunt-regulated dual power supply was designed by Paul Joppa, and utilized his latest thinking at the time. The Linestage is dead quiet, and it has plenty of drive for any headphones from the front panel output.

Cheers, Hank

 

[ToolGuyFred]

Very nice piece of kit, Hank.

I don't have the luxury of an active stage and after 100 hours of running in, despite more detail being resolved, I was still lacking some 'magic' in comparison to the active stage in the Foreplay it replaced. While considering the TVC, I measured the input resistance of the active sub. This started off the scale and reduced while the meter was connected - a sure sign of a capacitor on the input. Pulling the connections to the sub rewarded me with a return of the 'magic' but at the cost of the bass.

I just hooked up the sub via it's high level inputs, straight off the power amp speaker terminals. Job's a good 'un. I'm well pleased.

Next questions are:
1. Will this drive Paramounts? (Am borrowing a pair in a couple of weeks.)
2. Will a Reduction + Integration drive the TVC? (In build at present.)

Answers later.

 

[Paul Birkeland]

Pulling the connections to the sub rewarded me with a return of the 'magic' but at the cost of the bass.

I just hooked up the sub via it's high level inputs, straight off the power amp speaker terminals. Job's a good 'un. I'm well pleased.

You won't have much of a penalty if you feed the TVC's to your Foreplay and use it as a buffer between the TVC's and sub.

1. Will this drive Paramounts? (Am borrowing a pair in a couple of weeks.)
2. Will a Reduction + Integration drive the TVC? (In build at present.)

The Paramounts are a very high impedance load and don't have a ton of Miller capacitance, so you should be fine.  To answer question 2, you'd need to study the specifications of the Glasshouse TVC (which are lacking).

-PB

 

[ToolGuyFred]

You won't have much of a penalty if you feed the TVC's to your Foreplay and use it as a buffer between the TVC's and sub.

If I have to use the Foreplay as a buffer (and I don't think I will) then I'll just use the output stage. A Foreplay has way too much gain for my (original) ParaSEX monoblocks - I had to use custom resistor values in the sweet whispers to pad the inputs down.

The Paramounts are a very high impedance load and don't have a ton of Miller capacitance, so you should be fine.  To answer question 2, you'd need to study the specifications of the Glasshouse TVC (which are lacking).

-PB

I'm confident that using the Integration current sources will give the Reduction it's best chance to drive the TVC. If it isn't good then the Foreplay will be my "Get Out of Jail" card - even if its only for the extra gain.

 

[Paul Birkeland]

The padding resistors at the inputs of the Foreplay are actually beneficial for what you're wanting to do.  If the gain is bothersome, higher impedance padding resistors just present that much less of a load to your TVC's.

 

[ToolGuyFred]

Hi Paul

You are absolutely correct from a technical standpoint.

I am trying to replace the Foreplay rather than add to it for two reasons:
1. The switches for the stepped attenuator are now so noisy I have to select an unused source while I change the volume.
2. With so much (un-needed) gain, finding tubes which are quiet and remain quiet is an ongoing task. I share a modest house with three other family members and listen to a lot of music: this includes low volumes late at night when noisy tubes become really intrusive. Modern production are least satisfactory, (regardless of price) some go noisy in a matter of days, few have managed a year. The best I have had were three NOS (1960s production) Siemens tubes I scored for under 50 quid on fleabay. One of these has been in constant use for over three years. The other two have gone noisy.

So... If I need the gain for the Reduction, it will just have the Foreplay on that input and if I need the follower stage to augment the TVC I will use that. I hope I won't need either.

 

[Doc B]

If tubes and switches are going noisy on you I'm thinking you should start by using some DeOxit to clean the switch contacts and tube pins. Should probably hit the RCA jacks too. That will get the system resolution up to where you can make better informed judgments about the changes you plan.