One channel is alive! I Played it for a while then a LaScala. Initial impressions are that it is dead silent sitting on the bench, no hum.
On the S/500, and I assume other Threshold amps, the mounting studs for the front end board is a longer screw that goes through a output transistor case. I moved the long screw over one transistor. The boards that I used are the Nelson Pass boards with extra holes. None of the holes are meant to fit the S/500. I had to manually drill them and also make a bracket to adapt the other side of the board. See the picture. Everything is just mocked up until I get all of the details sorted. The channels was warmed up in the vertical position raised off of the desk about an inch. First pic was just a smoke test.
I mounted the trimmer pots to the back side of the board. I don't have the delicate kungfu trimmer pot turning abilities that Master Pass harnesses... So I used side adjust multi-turn straight pin trimmers. I drilled a hole in the center of the two outer pins and soldered the middle pin to an outer noting the trace. This allows you to flip the pots one direction or the other so you can use a long screw driver to adjust from the top if the amp with the cover off.
I put a heatsink on Q5 as it got hot. Q6 and Q7 got pretty warm but not hot. I would like to source the heatsinks that are in the original post
Initial Issues to resolve:
1) I cannot get the bias low enough. I was wondering if I should raise the value of R19 to maybe ~500 ohms or change out to a 1k pot. The old front end used to start with a higher bias that would lower as the amp came to temp. With the new, the bias raises as the temperature goes up... Bias used to settle around 60-80mv accross a 1 ohm source resistor. With the new front end, with the pot at its highest resistance, I got up to 54 degrees and 125mv and rising. I bypassed the thermostat for now. I will hook it up. Not excited to drill holes in the chassis but it's a good safety measure.
2) Also, the offset isn't bad but the pot only get's it down to 40mv. I am wondering if I need to refine the matching on Q3 and Q4. or change a resistor value. I used a cheapie transistor checker as I couldn't find a way to match like you can a JFET (using Mr. Pass's perscribed method). LSK170's are from the DIYstore and on my little transistor checker, they match perfectly as to be expected. Is there anything else that should be matched on the front end?
3) Square wave isn't very square. However, I don't have a signal generator. Can I expect a topping D90 to create an accurate square wave?
4) what are the heatsinks used in post 1?
On the S/500, and I assume other Threshold amps, the mounting studs for the front end board is a longer screw that goes through a output transistor case. I moved the long screw over one transistor. The boards that I used are the Nelson Pass boards with extra holes. None of the holes are meant to fit the S/500. I had to manually drill them and also make a bracket to adapt the other side of the board. See the picture. Everything is just mocked up until I get all of the details sorted. The channels was warmed up in the vertical position raised off of the desk about an inch. First pic was just a smoke test.
I mounted the trimmer pots to the back side of the board. I don't have the delicate kungfu trimmer pot turning abilities that Master Pass harnesses... So I used side adjust multi-turn straight pin trimmers. I drilled a hole in the center of the two outer pins and soldered the middle pin to an outer noting the trace. This allows you to flip the pots one direction or the other so you can use a long screw driver to adjust from the top if the amp with the cover off.
I put a heatsink on Q5 as it got hot. Q6 and Q7 got pretty warm but not hot. I would like to source the heatsinks that are in the original post
Initial Issues to resolve:
1) I cannot get the bias low enough. I was wondering if I should raise the value of R19 to maybe ~500 ohms or change out to a 1k pot. The old front end used to start with a higher bias that would lower as the amp came to temp. With the new, the bias raises as the temperature goes up... Bias used to settle around 60-80mv accross a 1 ohm source resistor. With the new front end, with the pot at its highest resistance, I got up to 54 degrees and 125mv and rising. I bypassed the thermostat for now. I will hook it up. Not excited to drill holes in the chassis but it's a good safety measure.
2) Also, the offset isn't bad but the pot only get's it down to 40mv. I am wondering if I need to refine the matching on Q3 and Q4. or change a resistor value. I used a cheapie transistor checker as I couldn't find a way to match like you can a JFET (using Mr. Pass's perscribed method). LSK170's are from the DIYstore and on my little transistor checker, they match perfectly as to be expected. Is there anything else that should be matched on the front end?
3) Square wave isn't very square. However, I don't have a signal generator. Can I expect a topping D90 to create an accurate square wave?
4) what are the heatsinks used in post 1?
