[Gluegun]

Yo, Kurt, coming down to Orange County any time soon? I know it's a drive, but... maybe we could meet...

Lol, I'd like to TRY the Corda Blue / DT931 one of these days...

And, that stuff you discussed with Jan, would that make the Blue even better? how much would it add to the cost?

 

[Jan Meier]

Oops, I thought I placed an answer to Kurt's questions but apparently didn't save/add it properly. Let's try again.

"Doesn't placing the buffers inside the FB loop interfere with the crossfeed circuit?"

No, the buffers are placed inside the loop of the output buffer only. The PREHEAD has the conventional topology of an input buffer and an output buffer with the filter in between.

"Regarding the LM334, this looks like a handy solution for biasing into class A. I noticed the capacitance is much higher than using discrete FETs like the 2N5485."

The transconductance capacitance of the LM334 is 15pF. for the opamps this is the same as driving an open ended wire with a length of 10~20 centimeters. No problem at all. At 10 kHz the effective impedance still is 1 MegaOhm!

Cheers,

Jan

 

[KurtW]

Hi Jan, thanks for your response. I'm a bit confused though about "the buffers are placed inside the loop of the output buffer only". Since the buffer has no inverting input, how do you create a loop? Are you adding a control op amp to do this, as your eariler post suggested? I'm not trying to get you to give away any secrets, but this was confusing to me.

Gluegun, I have no plans to visit Orange County as of now, but longer term I've been thinking about a short trip there; I'll let you know if it happens. The space is rather limited in the Corda HA-1 for doing too much more to the outputs. Jan has the benefit of starting with a new layout for the Prehead. I'm thinking about ways to get three buffers per channel though, so maybe I'll try that. I had thought two was overkill, but maybe not.

 

[kelly]

Is there such a thing as a secret in an amp that is available in kit form?

 

[Jan Meier]

"I'm a bit confused though about "the buffers are placed inside the loop of the output buffer only". Since the buffer has no inverting input, how do you create a loop? "

Very simply. The output of the opamp is connected to the input of the BUF634. The output of the BUF634 is connected through a resistor to the inverting input of the opamp.

Although most people tend to use the BUF634 as a unit-gain amplification stage it never was intended to be used like this.

Search the Burr-Brown internet site for "BUF634" and you will find an application note "combining an amplifier with the BUF634" which explains it all.

Have fun,

Jan

 

[KurtW]

Hello Jan, thanks for your comments. I have seen this app note but was confused if you were adding another op amp to do this or was using the one in the first stage, and if so if that affected the crossfeed. So it sounds like you are adding another op amp just to control the output buffer.

 

[buchignani]

Some time ago, KurtW queried: "Regarding the LM334, this looks like a handy solution for biasing into class A. I noticed the capacitance is much higher than using discrete FETs like the 2N5485, but I assume its still low enough not to be a problem."

Jan replied that the LM334 has only 15pF, and should not affect anything.

I recently started to further upgrade my Corda HA-1 (earlier: changed rectifiers, added power supply capacitance) in a bluish direction. The first thing I did was to replace the 1.5k resistor in the first stage that biases the LM6171 into Class A.

As an aside, I must say that regulation ability of the LM334 is amazing. Seems imune to load, and the variation in current as a function of voltage in the 2-15V range is around 0.1%

I tried 1 mA first and it really had a bad effect, especially on the highs. Moved to 5.5 mA and this was a great improvement. Still seemed as if the highs were a little muted. I then replaced the standalone LM334 with an LM334/single Jfet combination (2n5486) as per the LM334 data sheet. This reduces the capacitance to that of the jFet--around 3 pF. Can't say that the reduction in capacitance was the reason, but the result (at 4.5 mA) was the best of all the combinations I tried.

As another aside, I then added BUF634s in the output stage and then doubled them up. I would not have imagined that doubling the BUF634s would make much difference using rel. high impedance phones like the HD600, but it does. Bass solidity was markedly improved. By the way, I selected a 7 mA bias level for the BUF634s rather than the flat out 15 mA of the high bandwidth mode: the charts show essentially all the benefit is gained by that level of bias. Heatsinked the chips anyway, even though it wasn't necessary.

Norman