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
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
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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
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
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
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, 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
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
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
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
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)
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)
Series shunt , nothing has come close to the transparency that this gives, (Silonex part NSL-32SR2S Optocoupler (sorted)) i also then have to match 4 of them up, this is costly and time consuming. Give it a go you`ll never go back to a pot again.
As far as i`m concerned pots have no place in the signal path in high end audio.
The only draw back is you never get absolute zero volume, but that`s no problem.
Cheers George
As far as i`m concerned pots have no place in the signal path in high end audio.
The only draw back is you never get absolute zero volume, but that`s no problem.
Cheers George
Just a little bit of history on this Lightspeed Attenuator of mine.
I first made one of these 20 years ago, back in those days you had to buy the ldr and led separatly glue them in each end of a tube, the only problem back then i had to sort through 50 or so ldr`s before i got 2 to remotley track each other.
Then it got shelved (i got bored with hifi) till i read about Mark Porzilli of Melos fame had taken up the idea in his SHA- Reference Gold pre-amp in 1997, but even he had tracking problems, (lotsof returns) but everone who heard one of these working correctly said it`s one of the best sounds they ever listened to.
Then Melos went broke.
A year ago i found the Silonex ones we`ve come a long way since my originals 20 years ago, now the led/ldr is in one package and the matching while not great still is exceptable if your prepared to do some additional matching.
That`s the Lightspeed Attenuator story.(Mark Porzilli called his the Photentiometer) tricky name.
Cheers George
I first made one of these 20 years ago, back in those days you had to buy the ldr and led separatly glue them in each end of a tube, the only problem back then i had to sort through 50 or so ldr`s before i got 2 to remotley track each other.
Then it got shelved (i got bored with hifi) till i read about Mark Porzilli of Melos fame had taken up the idea in his SHA- Reference Gold pre-amp in 1997, but even he had tracking problems, (lotsof returns) but everone who heard one of these working correctly said it`s one of the best sounds they ever listened to.
Then Melos went broke.
A year ago i found the Silonex ones we`ve come a long way since my originals 20 years ago, now the led/ldr is in one package and the matching while not great still is exceptable if your prepared to do some additional matching.
That`s the Lightspeed Attenuator story.(Mark Porzilli called his the Photentiometer) tricky name.
Cheers George
George,
I have one question regarding this type of attenuator.
I own a Melos Gold and you're correct, it sounds very, very good...
BUT...
I'm paranoid about using it...
When the volume is set on the lowest setting, the light on the pc board is at it's brightest setting. As the volume is turned up, the light is dimmed...
My paranoia lies in the fact that if that light bulb were to burn out, the volume would immediately go to full and subsequently destroy my speakers....
Melos used what looks to be an interior dome light bulb from a car with the snap-in clips soldered onto the circuit board...
I does sound good though...
Any thoughts on this?
I have one question regarding this type of attenuator.
I own a Melos Gold and you're correct, it sounds very, very good...
BUT...
I'm paranoid about using it...
When the volume is set on the lowest setting, the light on the pc board is at it's brightest setting. As the volume is turned up, the light is dimmed...
My paranoia lies in the fact that if that light bulb were to burn out, the volume would immediately go to full and subsequently destroy my speakers....
Melos used what looks to be an interior dome light bulb from a car with the snap-in clips soldered onto the circuit board...
I does sound good though...
Any thoughts on this?
I would measure the voltage on the lamp with it set at its maximum to see how close it is being operated to its rated voltage. If its very close to the max then I would replace the lamp at regular intervals to ensure it doesn't fail. If you only get to about 80% of its rating it should last a very long time. In servicing movie theatre sound systems the exciter lamps that used to be used lasted on average about a year if run at 80% rated voltage and 8 hours a day. These were well made lamps though... much higher quality than auto stuff. If you still don't think that regular interval replacement is enough insurance then converting to an extremely bright white LED might be better insurance, although it may limit your control over the lower ranges somewhat todays LED's can be varied in brightness over an extremely wide range. If that won't work I would definately consider fusing the speaker lines till you work something out that you're comfortable with.
Mark
Mark
Steve, that was another probem, pity you live in the USA, i`m in Australia, i could subsitute those for the ones i`m using which are factory sealed (and track much better) in their own package and give 5years life at full power constantly on (24hrs a day).
You like the sound of the unit when it`s all warmed up naturally which cuts into the life expecancy of the the globe, what you could do is when leaving it on is to make sure the volume is not at zero, but say at 3/4 full, which will dim the lamp so it should last longer. Or (big hassle) you could get me to do the mods.
Cheers George
Btw do you get much channel drift, so you have to compensate with a balance conltrol.
You like the sound of the unit when it`s all warmed up naturally which cuts into the life expecancy of the the globe, what you could do is when leaving it on is to make sure the volume is not at zero, but say at 3/4 full, which will dim the lamp so it should last longer. Or (big hassle) you could get me to do the mods.
Cheers George
Btw do you get much channel drift, so you have to compensate with a balance conltrol.
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