A little more twisted than usual....

[Russ White]

1)These regulators, they power the power amps? The heat sinks seem way to small...




2) How does the balanced opamp as an input buffer improve performance? I imagine it can improve the S/N ratio but does it do more to improve sound quality? Perhaps it colors the sound in a pleasing way? (nothing wrong with that for hi-fi)

1) No, those power the TXD only, not the power stage.

2) The TXD (THS4131) "IS" the amp, not just a buffer. It actually forms the dominant symmetric feedback loop. The LM4562 can be looked at as a sort of buffer, but it is in fact an instrumentation amp. The benefit is extremely low distortion and high dynamic range coupled with very good PSRR. The amp also rejects common mode noise very well.

Cheers!
Russ

 

[BrianDonegan]

These regulators, they power the power amps? The heat sinks seem way too small...

1) No, those power the TXD only, not the power stage.

Incidently, it is possible to use the regulated section for amp power, but you need to mount the FETs under the board and mount the whole thing on a heatsink.

The FETs are STP80NF55-06/STP80PF55 from ST (55V/80A/300W). You can use other FETs for higher current if needed. We chose these for optimum impedance performance on the supply (<0.018ohm Ron).

 

[Pathmark]

Russ & Brian,

Thanks again for great kits, so far 1 orginal 3886 kit and now a TXO kit in standalone config, no TXD, working great. My questions is regarding buffer amp.

I have read many posts regarding the "improvement" with the use of buffer. Unfortunately, my attempts to build NUUK discrete buffer are not so succesful.

So my question is does the TXD fill this role? If yes do i need two, one for each channel?

Thanks,
Pathmark

 

[Russ White]

Hi Pathmark, and thanks.

Do you actually have a TXO? Or a pair of TPMs?

If you actually have a TXO it already has the TXD module included for each channel (it is key), and will not require any other buffer as the LM4562 on the TXD acts like a buffer.

Yes, you certainly could use the TXD as a balanced output buffer/line driver (input could be SE). With single ended output it would work fine but it would not be as optimal.

You could actually easily use a single TXD PCB as a single ended stereo buffer by omitting the THS4131 and placing resistors to alternate pads (I have actually done this).

But one word of advice. I bet the TPM would actually sound just great without a buffer, and perhaps better. The reason is that the input impedance is already quite high, and it does not require much current. So a buffer is kinda wasted. Inverting chip amps or low impedance chip amps are a whole different story. They need the extra current.

Cheers!
Russ

 

[BWRX]

The FETs are STP80NF55-06/STP80PF55 from ST (55V/80A/300W). You can use other FETs for higher current if needed. We chose these for optimum impedance performance on the supply (<0.018ohm Ron).

Are these FETs used as the pass element in a linear regulator?

 

[Russ White]

Are these FETs used as the pass element in a linear regulator?

Yes this is true. It just one factor in the result. The actual output impedance is still plenty low, but in fact the design of the TXD does not actually require low output impedance.

Cheers!
Russ

 

[BrianDonegan]

Reading that, it was worded poorly. I did not mean to imply the the Ron was the impedance of the PS.

 

[Pathmark]

Russ,

THanks for reply, yes I have TPM modules (2). So are you saying that in current stand alone config no buffer is required?

Mike

 

[Russ White]

Russ,

THanks for reply, yes I have TPM modules (2). So are you saying that in current stand alone config no buffer is required?

Mike

No problem. No you should not need one at all unless your source has very high output impedance.

You could actually just use a 10K pot in front of the TPMs for volume control.

Cheers!
Russ

 

[BrianDonegan]

I think we just found a problem.

The schematic, as well as the part label, states that R11 should only be used with the TXO. This is not the case. It should be used in all configurations, giving a 100K input impedance.

We are making the needed updates to the docs. Sorry.

 

[Pathmark]

Brian,
Not smart enough to know this would be a problem I have been using w/o R11. I am using a passive volume control (autoformer type) being fed from a DAC.

What problems should have been apparent w/o R11? What changes should I expect w/ R11?

Pathmark

 

[Russ White]

Brian,
Not smart enough to know this would be a problem I have been using w/o R11. I am using a passive volume control (autoformer type) being fed from a DAC.

What problems should have been apparent w/o R11? What changes should I expect w/ R11?

Pathmark

Actually probably none. The input impedance of the amp would just be very high, which may not hurt anything.

Adding the 100K resistor can help reduce line noise if you have long lines to the amp. In your situation you may be best just to leave it out if you are not observing any issues.

Cheers!
Russ

 

[MashBill]

Russ and Brian,
I have have been following this thread with interest, but I too am confused about the various modules you offer. Maybe you could put some block diagrams on your website showing what modules would be needed for a preamp, amplifier, integrated amplifier, standalone DAC, etc.

I have a few other projects in the works right now, but when I finish them I want to build a standalone Opus DAC.

Bill

 

[BWRX]

The actual output impedance is still plenty low, but in fact the design of the TXD does not actually require low output impedance.

Most low current draw circuits with high PSRR don't require low output impedance, but it doesn't hurt either.

I did not mean to imply the the Ron was the impedance of the PS.

That's all I was getting at I couldn't find a schematic of the supply so I had to ask.

 

[BrianDonegan]

Most low current draw circuits with high PSRR don't require low output impedance, but it doesn't hurt either.

Yup. The TXPS is meant to be a general purpose supply, usable for a variety of applications. High current unregulated, medium and low current regulated, and various combinations of the two.

 

[BrianDonegan]

I have have been following this thread with interest, but I too am confused about the various modules you offer. Maybe you could put some block diagrams on your website showing what modules would be needed for a preamp, amplifier, integrated amplifier, standalone DAC, etc.

Keep trying to get to this, but there are never enough hours in the day. I will try to get this done this weekend, as it's been bugging me for a while.

 

[SheldonD]

Hi Russ I have further questions re TXD

1. I have 2 sets of TXD boards with which I wish to drive two amplifiers:

a. TXO-2 with LM3886

b. TXO-4 with LM4780

the input for each amp will be single end passive preamp output. I believe the answers you give will apply to both amps.

The amp modules are from Chipamp.com and I will have to add the servos.

2. From your documentation and my previous questions the preferred gain at the amp modules should be 10.

3. the gain as the input TXD will be SE so what should the resistor values be to maintain a "line-level" input/output?. Secondly may it be optimum to rebalance the gains with respect to TXD and the amp modules.

4. As the TXD will be both: a. SE to balanced AND b. preamp
some of the statements regarding C7.C8 , the Jumpers and the gain values set by resistors etc seem contradictory.
I am more than a bit confused.

I am suffering from "paralysis thru analysis"

5. TXD ICs are both run with +/- 10 Volts

Thanks,

SheldonD

 

[Russ White]

Hi Sheldon.

Sound like a couple fun projects.

You seem to be thinking exactly down the right lines.

Here are the important points:

The gain of the TXD sets the gain of the entire amp, and with single ended input(-IN tied to GND) the output will be 6db less than with balanced input(with the level).

So lets use 2Vpp inputs as an example. With balanced input that would be doubled so 4Vpp. 20db gain is 10X voltage gain. 10 X 4vpp is 40Vpp(as a reference)

To get that same gain with an equivalent SE signal you will want to add 6db of gain. That works out to 26db or 20X voltage gain.

Now RF and RG have been chosen along with C7/C8(compensation) for stability. So I would not adjust these, instead its simpler to adjust R1 and R2.

G = ( RF / RG ) * ( 1 + (2R2 / R1) )

Choose R1 and R2 for a factor of 2 (R1= 4.7K and R2= 4.7K should be fine).

1 + (4.7K / 4.7K) = 2

Make sure R1 and R2 resistors are as well matched as possible. At worst 1%.

10V rails is just fine.

I hope this helps you get up and going.

Please share your results!!!

Cheers!
Russ

 

[Russ White]

Also, now that I look at my own schematic I remember that the recommended values are:

RG=2.2K RF=22.1K

You could easily get the same 6db boost for SE input by halving RG to 1.1K, 1K is certainly fine here too.


That would get you the same ~ 20V voltage gain as adjusting R1/R2 as in my previous post.

I still like my first suggestion best (because the interstage impedance remains unchanged) but either approach should produce excellent results.

For TXO with RF=22K C7 and C8 need to be at least 220pf. You can go up to around 330pf with no audible penalty.

Cheers!
Russ

 

[BrianDonegan]

And to come full circle, the TXD (as well as the TXD kits within the larger TXO kits) kit comes with RG of 2.21k - 0.1% (standard value - 10X gain) as well as 1.1K - 0.1% (for 20X gain).