Adding extra attenuation to S&B TX-102

[mlvp]

Hi,

After years of simply listening to audio equipment, I found out that building/tinkering adds a lot of fun. Therefore I am considering a Bent Audio NOH or Django transformer volume control kit based on the Stevens & Billington TX-102. But I am a complete beginner; perhaps someone could offer advice.

The trouble is, I need more attenuation than the TX-102 can provide. The output of my CD player is 4.6V (balanced outputs); the gain of my amplifier is some 29dB, and I listen at low/very low levels (less than 1V at the speaker terminals).

Although Bent Audio offered to build a custom TX-102 (with more attenuation), I have more or less decided against it. The standard version would be more flexible when changing my system. However, this means that some add-on attenuation (12dB) is needed.

I found two commercial options: a 4:1 step-down transformer from Jensen (JT-10-KBD), or external attenuators from Rothwell Electronics.

The Rothwells use a voltage divider network, which sounds simple enough to build it myself. Some questions:

• Would there be much difference in sound between the transformer (the Jensen) and the resistors?
• I would like to use the balanced inputs; does this require precisely matched attenuation for both polarities?
• What resistor values should I look for? (The input impendance of the amp is 220K.)
• Do I place the attenuators at the input of the passive, or at the input of the amp?
• Anything else I overlooked?
• Did I use the correct forum for this question?

Thanks in advance,

Michiel

[Steve Eddy]

No chance at all of having the amplifier modified to reduce its voltage gain?

se

[ThorstenL]

Konnichiwa,

Originally posted by mlvp
The trouble is, I need more attenuation than the TX-102 can provide. The output of my CD player is 4.6V (balanced outputs); the gain of my amplifier is some 29dB, and I listen at low/very low levels (less than 1V at the speaker terminals).

Well, the latest (MK III) 102 has attenuation all the way down to -52db, it should actually suffice in your situation, as it would give around 0.32V RMS on the speaker terminals or around 20db below 1W. If your speakers have a voltage sensitivity of 95db/2.83V/1m you will have peak levels of around 76db and average levels with most music of 66db. This is a VERY LOW volume to listen to music.

My current "in system" amplifiers have a gain of 34db and my CD Player has an output of 2V RMS. There ano attenuation problems using the latest 102, even for late nigh listening, my speakers are 95db/2.83V/1m Sensitivity, for reference.

So I think you may not need any added attenuation.

BTW, if your CD-Player is balanced in the way that is sadly common in consumer audio (meaning the output voltage is double that of RCA operation and thus not really balanced but double single ended) I think you may find a single ended feed to sound better anyway, try it.

Originally posted by mlvp
[*]Would there be much difference in sound between the transformer (the Jensen) and the resistors?

Likely.

Originally posted by mlvp
[*]I would like to use the balanced inputs; does this require precisely matched attenuation for both polarities?

Yes. Or place the attenuation on the input of the Poweramp. Use a pair of closely matched resistors of 3k9 in each polarity and a 2k7 resistor for a balanced input attenuator for the Amp or 8k2 & 2k7 for unbalanced.

Sayonara

[mlvp]

No chance at all of having the amplifier modified to reduce its voltage gain?

I guess I could have it modified by the dealer I bought it from, they certainly have the expertise and the links to the manufacturer. In fact, they modified the pre-amp for lower gain when I bought the set.

I would have to ask for a quote, but if the price of an available upgrade for the amp is any indication, it may be the most expensive option. (Electrocompaniet asks 500 Euro for an upgrade to my AW-100 DMB providing "improved linearity and speed in the power supply and output stage".)

Michiel

[Peter Daniel]

I'd say get your transformers first and see how they work. If you have too much gain in the amp, just place a resistive divider at the input (of the amp); shouldn't cost you much even when done with the best resistors.

[Steve Eddy]

Originally posted by mlvp
I guess I could have it modified by the dealer I bought it from, they certainly have the expertise and the links to the manufacturer. In fact, they modified the pre-amp for lower gain when I bought the set.

I would have to ask for a quote, but if the price of an available upgrade for the amp is any indication, it may be the most expensive option. (Electrocompaniet asks 500 Euro for an upgrade to my AW-100 DMB providing "improved linearity and speed in the power supply and output stage".)

Do you have any interest in the upgrade? If so, perhaps they'd be willing to change the gain for no additional charge.

If reducing the amp's gain isn't a practical option, Kuei's suggestion of using the unbalanced outputs on your CD player would be next on my list of suggestions.

If your CD player's going to be driving the TX-102, feeding it from a balanced source is of little advantage in terms of common-mode noise rejection.

Transformers are rather unique in that they can provide outstanding common-mode rejection even when fed from a wholly unbalanced source. Better than most electronically balanced inputs can achieve when they're fed from a balanced source.

I've been using input transformers for years and my preference has been to feed them from unbalanced sources.

Anyway, the 6dB reduction you get from using the unbalanced outputs may be sufficient for you.

se

[Steve Eddy]

Originally posted by Peter Daniel
I'd say get your transformers first and see how they work.

Only if he can return them if they don't work out for him. The Jensen 10KB-D goes for $70 a pop over here in the US.

He might want to look into an equivalent from Lundahl or Sowter if he chooses to go with the transformer option. They tend to sell for rather less than the Jensens.

se

[mlvp]

Well, the latest (MK III) 102 has attenuation all the way down to -52db, it should actually suffice in your situation, as it would give around 0.32V RMS on the speaker terminals or around 20db below 1W.

True, but if it has the same tap spacing as the Mk II, the next step up is 6dB louder. Ideally, I would like one or two steps between them.

My current "in system" amplifiers have a gain of 34db and my CD Player has an output of 2V RMS. There ano attenuation problems using the latest 102, even for late nigh listening, my speakers are 95db/2.83V/1m Sensitivity, for reference.

My speakers are only 86dB sensitive, so I listen at very low levels indeed. I live in a small, slightly noisy apartment building and the thought of sound leaking to the neighbours makes me back off the volume. Still, the actual numbers suprised me. The system is too sharp/edgy, so perhaps it sounds louder than it is (I should get an SPL meter).

When I read your experiences, Peter Daniel's suggestion to build the kit and see what I need seems wise.

BTW, if your CD-Player is balanced in the way that is sadly common in consumer audio (meaning the output voltage is double that of RCA operation and thus not really balanced but double single ended) I think you may find a single ended feed to sound better anyway, try it.

It is indeed! In fact, I thought that this was the very definition of a balanced circuit (this opposed to a single-ended circuit with a phase splitter). Obviously, you seem to have your reservations... would you mind to explain them a bit (or point me to an explanation)?

Thanks,

Michiel

[ThorstenL]

Konnichiwa,

Originally posted by mlvp
True, but if it has the same tap spacing as the Mk II, the next step up is 6dB louder. Ideally, I would like one or two steps between them.

If you really listen that far down 9as opposed to late night quiet listening) I would suggest that you have way too much gain, not just 12db too much.

Originally posted by mlvp
It is indeed! In fact, I thought that this was the very definition of a balanced circuit (this opposed to a single-ended circuit with a phase splitter).

A truely "balanced" circuit will have the same gain and output voltage regardless of unbalanced or balanced termination. Classically this is attained by using transformer coupling and completely single ended signal circuitry in Pro-Audio gear (where balanced connections have their origin and application).

In recent years electronically "servo balanced" inputs and outputs have replaced them in most Pro-Gear. Sadly, as it seems in "high end" there is still a completly irrational and unjustified prejudice against using transformers and pro-audio style circuitry is also generally frowned upon (for more realistic reasons I'd add) you get a circuitry that usually makes a "balanced" output by splitting a single ended output up or by running two parallel circuit path with opposite signal polarity. In either case it usually sounds better to use single ended connections, once the level differences are removed.

Sayonara

[mlvp]

Hi,

Thanks everyone for your responses. A few last remarks:

Steve Eddy wrote:

Do you have any interest in the upgrade? If so, perhaps they'd be willing to change the gain for no additional charge.

If reducing the amp's gain isn't a practical option, Kuei's suggestion of using the unbalanced outputs on your CD player would be next on my list of suggestions.

I'm not really interested in the upgrade, so I will take the advice and get myself an unbalanced interconnect for the CD player. I'll add resistors if needed.

Kuei Yang Wang wrote:

If you really listen that far down 9as opposed to late night quiet listening) I would suggest that you have way too much gain, not just 12db too much.

I know... the 12dB was meant to make the TX-102 work in my current system, but I really need to rethink my amplification. (In fact, I don't need any amplification at all, a buffer stage would be fine.) But the TX-102 kit will get me started.

Arigatou gozaimasu/Thanks a lot,

Michiel