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After almost three and a half months, the KGST is finally in operation, and is sounding great. Thanks to all who have responded to my questions and given guidance. I am really surprised at  how good t

Thanks to the NYC Spring Meet yesterday, I was motivated to finish casing up my KGST.  I'm very happy with the result.  The chassis is 7" x 12.8". Perfect for my night table   Here's the beast in i

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On 10/6/2020 at 11:18 AM, matthew-levi said:

I could not adjust the bias down to 230V (for my normal bias Lambda). The lowest I could adjust is 333V.  How should I modify the voltage divider circuit to all the voltage to go down further. 

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At the power supply schematic – shouldn’t D12 anode be connected to the cathode of D1?

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This is the schematics I downloaded from the web. On the board, I think the D12 anode is connected to the cathode of D1. So the board is correct, but the schematics may have an error (at least on this version that I managed to lay my hands on).

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What should be the upper limit of the off-set  ( between O+ and ground). It fluctuates from a couple of voltages to 100 mV range. This is after the amp has been running for about half an hour. When its so unstable, does it mean something is not quite right. One of my friends told me that the off-set should be in the 10s of mV range. 

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how is the voltage O- to ground does it behave and vary similarly to the O+? 

are both channels approximately the same?

 

Unless there is a servo to automatically adjust the offset then you will see some variation with time as the device heats up and reaches a stable temperature. My T2 takes more than 3 hours for the offsets to become mostly stable. Even with a servo there is a tendency for some low frequency oscillation since the servo will alter the current flow and the change in current current flow causes a change in the temperature of the components and temperature change effects the gain and therefore makes the correction over or undershoot - a servo is always chasing its own tail.... Plus unless your valves have regulated heaters, mains voltage varies with time and the variations in the ac heater voltages will affect the valve temperatures and therefore offsets too.

You need to put the offset into context.  The stax headphones are built to withstand 100s of volts of input. The music signal is going to be far far higher than this offset. For example the T2 is capable of more than 600V RMS output into pro bias headphones... even at very moderate listening levels of say 50Vrms output, 0.1V offset is a tiny fraction of the signal...

It's also normal for the offsets to be fairly high (10s of volts) on initial switch on, fairly rapidly decreasing while the valves initially warm up, especially if you do not have a delayed high voltage switch on circuit. 

Edited by jamesmking
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3 hours ago, matthew-levi said:

It fluctuates from a couple of voltages to 100 mV range.

To me it sounds that you built yourself a nice amplifier. Enjoy.

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11 hours ago, jamesmking said:

how is the voltage O- to ground does it behave and vary similarly to the O+? 

are both channels approximately the same?

Yes, both O+ and O- behave similarly, and both channels behave similarly too. For the balance i manage to get it down to around 100mv or less. 

I will do more testing and run it longer to see how they go. Right now I feel better knowing its should be ok. 

Thanks for the guidance. 

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  • 3 weeks later...

After almost three and a half months, the KGST is finally in operation, and is sounding great. Thanks to all who have responded to my questions and given guidance. I am really surprised at  how good the 40 year old headphone sounds with this amp. 

Special thanks also to Bigir and Dr. Gilmore for such wonder contributions to the community. 

 

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