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Would Volt regulator IC contribute harsh sound(IMD) ?

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Posted on February 8, 2017 at 19:45:30
Eric Chan
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Posts: 84
Joined: October 31, 2001
I had the following post regarding harsh sound existing on the BAT 6h30+6c33c amp, harsh sound go away after 2.5hour warm up.

Lot of advice said it should have taken that long to warm up, possibly some other peripherial circuit, I narrow down to input stage PS regulator and recitfier, but not sure my next action to experiment the replacement for Regulator IC or rectifier worth the effort

Please share your experience

 

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RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 8, 2017 at 22:39:54
drsx
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Most IC regulators do a poor job of regulating across the audio band.

 

RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 9, 2017 at 04:25:49
Stuben
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Posts: 513
Location: Guber Ohio
Joined: December 30, 2005
Eric,

You might consider having an over all look at your system. Gain,impedance matching between Pre and Power Amp, damping factor between your Power Amp and Speakers...etc.

The last 6c33c amp I heard was a BAT and it was not well matched with the other components of the system, listening fatigue was almost mediate. The 6h30 is an excellent voltage amp when well applied. You have a great amp there.. give it a chance to perform. The chances of actually having an issue with the amp is fairly low compared to a system dynamics issue. IMHO

Stuben




 

RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 9, 2017 at 09:40:47
Triode_Kingdom
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"I narrow down to input stage PS regulator and recitfier"

What did you uncover that leads you to believe the PS is the cause?


--------------------------
Buy Chinese. Bury freedom.

 

It can, but, posted on February 9, 2017 at 11:30:50
Ralph
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the 6H30 can be a source of that too. I bet these are easier to deal with :)

If the regulator is poorly bypassed, you can run into HF artifacts.

The regulator has a certain amount of bandwidth. Above that frequency it should be bypassed with a capacitance (so you can see here that the value of ideal capacitance may not be all that large) such that the output impedance of the combination is as linear as possible (there will be a step in the curve as the regulator typically has lower output impedance than the bypass cap will).

Hopefully that's all been dealt with. But what if the cap is failing, like maybe its a small electrolytic and just takes a while to come up to speed?

IOW it might be worth changing that part if this is the case but I would be hesitant to change out the regulator without doing the math.

 

RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 9, 2017 at 14:12:09
Stuben
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Eric, Sorry, my memory had a hickup...the 6C33C amp that referred to was a LAMM product.

Stuben

 

RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 9, 2017 at 15:30:17
Michael Samra
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I had the following post regarding harsh sound existing on the BAT 6h30+6c33c amp, harsh sound go away after 2.5hour warm up.

In this case, the only thing that will cause that is tubes or bias.The 6H30 is definitely suspect but what may be happening is the 6c33c tubes are coming into balance after a long warmup.They are probably mismatched on initial warmup. Keep in mind that harshness is a form of high frequency distortion so I would try tubes first.


"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Would Volt regulator IC contribute harsh sound(IMD) ?, posted on February 10, 2017 at 03:18:57
Eric Chan
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Posts: 84
Joined: October 31, 2001
It is because I have changed all the tubes and situation remain unchanged. If the harshness come from another place apart from tubes. The remaining suspects would be the PS. Just a logical assumption

 

RE: It can, but, posted on February 10, 2017 at 03:24:38
Eric Chan
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Posts: 84
Joined: October 31, 2001
I might try to simplfy the input stage PS by removing the regulator ICs, both positive and negative rails for the first differential stage first, by swapping into the PI RC PS to see if the harsh would go away.


Second thought is to swap in a 6H6n tube to replace the 6H30, this move may need to change the valuse of 1st stage resistor values because the 1st and 2nd stages are direct coupled

 

RE: It can, but, posted on February 10, 2017 at 11:13:18
JKT
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"Second thought is to swap in a 6H6n tube to replace the 6H30, this move may need to change the valuse of 1st stage resistor values because the 1st and 2nd stages are direct coupled"

Without seeing the schematic this is only a guess. If the 1st and 2nd stages are differential and direct coupled check to see if there is a voltage differential between the two grids of the 2nd stage. Any voltage difference there will "eat" into the voltage headroom of the second stage. If this condition were found to be worse at start-up that could explain the problem. If this is the case changing to 6N6Ps would make it worse. At the same plate voltage and bias current the grid bias voltage of the 6N6P is about half that of the 6N30 (less headroom).

I have a fully differential phono pre where the first and second stages are direct coupled. The first stage is a MOSFET/tube cascode with CCS plate loads (very high output impedance). Some of the plate load current source current is shunted to ground across a resistor (part of the RIAA network). This sets the plate voltage of the 1st tube and the grid voltage of the second. Matching this voltage for the two halves is very important. When they are not matched this does result in a hard sounding distortion. The second stage of this preamp is a 6N30 (150Vp/k, 25mA/side). Originally I used a 6N6P. In this application the 6N6P was more prone to brittle sounding highs and harsh transients. Your milage may vary



"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

One of the problems with this design, posted on February 10, 2017 at 11:35:33
Ralph
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-Is the direct-coupled fully differential circuit.

FWIW Victor had a set of our amps in front of him and were the inspiration for the VK-60, which was the first amplifier BAT made after he and Steven Bednarski (a customer of ours) went into business together.

Unbeknownst to Victor (who to his credit did not copy our circuit although he did use the Circlotron) some years earlier (about 1985) we had tried and rejected the driver circuit he used in the VK-60 (this driver topology was also used by Tenor a few years later). The problem with it is accurately described in JKT's post next to this one (and I believe his explanation for your problem is also accurate- you need matched tubes in the driver for this to really work right). A balancing pot could be used in the cathode circuit of the first gain stage; the problem we found is the pot causes as many problems as it solves. We never got this circuit to sound right and by all accounts I have ever heard neither did Victor (nor Tenor for that matter).

IMO, you are better off having an additional set of coupling caps between the first and second stages to avoid the offsets (and distortions) that otherwise occur. Easier said than done of course- that would require a bit of doing!

 

RE: One of the problems with this design, posted on February 10, 2017 at 12:58:32
JKT
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I have fully differential mono-block power amps where the 1st and 2nd stages are direct coupled. Voltage offset in the output of the 1st stage is avoided by the use of a CT plate choke and a reasonably balanced 6SN7 (necessary for the choke) for the first stage. Low DCR of the choke and small imbalance current take care of that.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: One of the problems with this design, posted on February 11, 2017 at 08:47:58
Eric Chan
Audiophile

Posts: 84
Joined: October 31, 2001
Hi Ralph and JKT,

>> who to his credit did not copy our circuit although he did use the Circlotron) some years earlier

I think that all BAT 6c33c amp use typical plate load OPT coupled rather than Circlotron circuitry. and VK75se is completely no global negative feedback from 1st stage to final(To my understanding, Circlotron utilize output tube cathode to connect opt, correct me if mistaken)

A quick look on VK75se, there are two differential stages, 1st to 2nd are direct couple, then 2nd stage(paralleling 3 section 6SN7) cap couple to 6c33c grid, 6c33c plate load to OPT, quite a typical circuit that I always find similar schematic in MJ magazines.

As far as I know for the direct couple, the head room allow the 1st stage to swing on the 2nd stage grid is limited on the bias voltage difference applied on the 2nd stage grid to cathode. For the headroom allowed to swing on 2nd stage grid(both grids), the conservative 0.7 of amplification factor of 2nd stage x grid swing headroom seem more than enough to get second stage to produce 80-90 V p-p to drive the 6c33c into max power OP.

for sure I will give it a check to see the grid voltage of both 2nd stage grid voltages being equal.

One thing I do not understand is why you would say the balance pot used for differential stage cathode is problematic

Also if there is slight difference in tube match for differential stage, would it potentially produce unwanted IM distortion significantly?

 

RE: One of the problems with this design, posted on February 11, 2017 at 10:44:28
JKT
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Posts: 454
Location: Midwest
Joined: November 26, 2002
Let me give an example to clarify what I was saying. Suppose the driver tube (2nd stage) required a grid bias voltage of -6V at its operating point. As a first approximation this would limit the incoming signal to 6V peak to peak. If we're talking about a diff. pair, as an example let's say that the grids are offset by 3V. Then this condition would cut that 6V envelope in half and incoming signals greater than 3V p-p would distort. This is what I meant when I wrote that it would reduce headroom.

An imbalance in plate currents in the 1st stage diff. amp will result in different plate voltages (depending on the DCR of the plate loads) for the pair. Since the 1st and 2nd stages are direct coupled this difference will be expressed on the grids of the second stage.

If this proves to be the problem you will need to replace the input tube with one that has matched sections as measured in circuit.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: One of the problems with this design, posted on February 13, 2017 at 14:39:41
Ralph
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Location: Minnesota
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I think that all BAT 6c33c amp use typical plate load OPT coupled rather than Circlotron circuitry. and VK75se is completely no global negative feedback from 1st stage to final(To my understanding, Circlotron utilize output tube cathode to connect opt, correct me if mistaken)

The VK-60 uses an output transformer in a Circlotron output circuit. It is not a 'typical plate load OPT'.

 

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