Dear Russ,

Most likely, the Sympatico with Volumite is connected direct to an I/V output or CD player.

I'm considering to buy the Sympatico. But, can I add a preamp in between because I got other analog sources to select? Does it work fine? Does the gain of Sympatico need to be reduced?

Beisdes, when will the Ventus be in stock again?

Thanks a lot.

Thanks a lot, Dingo.

Sorry, the Volumite I said means attenuator (I should include the JT).

I'm looking for a true balanced bridged amp/chipamp. I think this kind of configuration has good noise rejection and the speaker output is lifted from the ground. That's why I'm here.

I'm a computer programmer knowing not much about electronics. Please help me out.

When I look at the schematic, I'm not sure whether the THS4131 has a x12 (1+22K/2K) gain. And, I think LME49720 is used for input impedance with x2 (1+2x1K/2K) gain.

From the datasheet of LM4780, Sympatico has been configured as a x11 (1+10K/1K) gain per channel in bridge mode. Ideally, with CD Player 2V input and 8ohm load, the output power is already 60W ((2Vx11)^2)/8ohm.

If THS4131 has a x12 gain, is it acting like a x10 line stage (preamp)?. If yes, I may not use my preamp any longer because the output may clip.

Do I have any misunderstanding to the circuit? As a diyer, we should learn something when building the kit.

Thanks, Dingo.

jackietang,

For one, I've never had a good look at the Sympatico until today lol, but i should be able to help you a bit ;)
You're pretty close with your maths, but also a bit off. So for the sake of educational purposes i shall try to explain.

For one, when you're looking at the gain stages in series with one another, you don't add/subtract, you should multiply. So when you subtracted 11 from 12, this wasn't right. You're probably getting confused with when you're expressing gains in dB, which because it is a logarithmic scale, you "add" in order to multiply. So consider a hypothetical x2(+6dB) gain stage followed by a x4(+12dB) gain stage. The overall gain is x8(+18dB).

If you look at the nested feedback loop, It is comprised as an inverting stage, followed by a non-inverting stage. Overall this will count as an inverting stage. A -ve gain x +ve gain = -ve gain. For an inverting stage the gain is calculated as Rf / Rs only, you don't add the +1 like you would for a non-inverting amp. So this sets the overall gain at (-)x11. You were right about the gain of the LM4780 being x11, and this leaves the THS4131 as a (-)unity stage.

As for the input Stage, with R10 omitted, it will just be a unity buffer for each +/- input, but with R10 in there you'll have a gain of x2 taking the overall amp gain up to x22. You would certainly want R10 there if using a SE input, to setup the differential input to the next stage.

As for why the THS4131 is there? Well i can't really give Russ' reason for his design now can I...
But if you didn't have it there, it wouldn't be a differential amp, but 2 completely seperate amps, one for each half of the balanced signal. So would end up looking just like a bridged amp, instead of a "real" balanced amp. Having the amplifier as a differential amp will give better CMRR and even harmonic distortion cancellation than would otherwise only be occurring across the speaker load itself(if the amp was merely bridged).

Let me clarify a few things...

the OPA1632(or THS4131) is not a buffer. It is the amplifier. :) The LM4780 is in fact the buffer, but one with gain. The only reason to give it it gain it to keep it operating it its stable region. The dual opamp in front of the OPA1632 simply serves as a instrumentation amp to increase the input impedance. this happened to be a convenient place to add a little gain for people who may use the Sympatico single ended.

The gain of the Front end and then the OPA1632 set the gain of the entire system. You can ignore the gain of the LM4780.

Some say they are the same chip, but they have very different costs, and I think the OPA sounds better.




Yes, I ship Bergquist K10 thermal pad with thee amps.



Soon. It is undergoing a tweak/redesign.

Hi Russ,

Is R10 necessary for SE input? I thought R10 was just to control the gain.