Tech help needed Threshold S/300 MkII optical bias

[georgehifi]

Hi guys, i need help with this amp it has 77k input impeadance and i need to up that to 200k, is it possible by just changing the resistor that goes across C3 i think it`s r18 or 10, without upseting the dc offset or bias or anything?

Also can anyone tell me what the switch is to the right of that resistor is for?

The reason for uping the input impeadance is for the high source impeadance that i`m driving it with, and it`s also in a bi-amped setup.

Nelson if you read this can you help please, after all you designed it.

And thanks to Dave (planet 10) for shrinking the circuit to the right size.

Cheers George

[audiousername]

Hi George,

Yeah, I think it's R18. It's value would probably be just a little less than 77K (maybe around 70K or so).

77K is already quite high... If your source has a very high output impedance (valve preamp?) it might have trouble driving the input capacitance of the FET, compromising high frequency response. But then I have no idea what I'm talking about when it comes to SS...

What is the output impedance of your source?

Jason

[georgehifi]

Hi Jason, i just measured the input rca that gave me the 77k the other resistor in series with the input could be making up the difference, the amp is only being used on the bass of my Martin Logan Monolith III`s it crosses over at 125hz so there`s no worry about the highs being rolled off.
My source is my own Passive volume control called the Lightspeed Attenuator. it consists of highly matched led/ldr packages and gives the source cd or da converter a constant load impeadance of around 10k but the output impeadance is high around the same as the input, hence the need for high input impeadance power amp, also this passive pre drives my 805 se monoblocks which are ok because they are 400k input, see thats why i need to up the input impeadance on the Threshold besause the two amps are being driven by the same passive.
I have a unity gain buffer for the Threshold which is 400k but i`d love not to use it and drive the Threshold without it.

Cheers George

[peranders]

First: What kind of signal source do you have with requires more than 77 kohms in load?

Do you want 200 kohms DC resistance of 200 kohms AC impedance?

R13, R18 and C3 may be changed.

Where do you have your rolloff, 50 kHz, 100 kHz?

[georgehifi]

Peranders, thanks for the reply, i was begining to think i`ve scared everyone off, i need the 200k input impeadance because this amp is the bass amp of a bi-amped system it`s being used from 125hz down, the main amps are my set805 monoblocks which have an input impeadance of 400k.
The problem i have is that my pre which has to drive both amps is 10k output impeadance, using a unity gain buffer i made that has 200k input impeadace and low output impeadance on the Threshold amp is keeping the bass nice and tight, if i remove it the pre is doing it hard into the 77k plus the 400k of the 805`s, hence the need to up the input impeadance of the Threshold to 200k or more.
I belive that getting rid of the buffer and driving it direct so long as my pre does`nt get loaded up i`ll have even better bass by getting rid of the buffer.

Cheers George from Oz

[SY]

George, with a FET input like this one, you can make that input resistor as large as you want (within reason).

[georgehifi]

Thanks SY, I thought that would have been the case but was`nt sure. If it were bipolar input the dc offset probally would of gone crazy right?
Have you any idea what that switch is for to the right of that input loading resistor?

The circuit diagram took me one year to find, seams to be the only one in existance, now it`s available for all to see, not even Nelson had it, all his others can be found but the Stasis S/300 and 500 MkII opticals were never let out, maybe Nakamichi kept them hidden.

Cheers George

[Tube_Dude]

Originally posted by georgehifi
Have you any idea what that switch is for to the right of that input loading resistor?

When the switch is open ,the amp don't work , because the CCS are not working.

Is the first time that I see a switch at this place...

[Mark A. Gulbrandsen]

georgehifi ,
You might wanna post this over at Pass Labs to be sure Nelson sees it.

Mark

[georgehifi]

Already have Mark, there`s more traffic through here, and i think Nelson is on leave or something.

Anyhow i took the plunge and did it it`s now 330k, with no change to dc ofset or bias, and wow was it worth the exercise now the bass is taught and extended without the need of the buffer.

I`ve done this exercise now on many tube, tranistors and buffers and i`m a firm believer in the 1 to 20 minimum ratio for output impeadace of the source verses the input impeadance of the next stage.

In my case i`m using my own Lightspeed Attenuator which consists of matched led/ldr components, it blows away any volume pot that i`ve tried (Alps Black Beauty, Penny@Giles, Alps Blue Velvet, and Bourns) Call it pot wiper distortion or whatever, but when the Lightspeed`s in there`s no wiper in the sigal path and you can hear it. The only problem is the high output impeadance and if you use the minimum 1 to 20 ratio in my case output of the Lightspeed is 10k the input of the poweramp has to be 200k or greater, the you hear magic.

Cheers George

(and thanks SY and all for giving the additional nod to change the Threshold`s input impeadance)