kgst

[matthew-levi 14]

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. 

[JoaMat 1,171]

If you remove C2 (0.1uF cap) I guess it will possible to set BIAS to 230V with help of the 1M trimmer.

[matthew-levi 14]

Hi JoaMat, I followed your advice and I have been able to set the bias to 230V. Thank you so much.

For the balance and off-set, I have been able to set them to around 1V, but it fluctuates quite a bit. I think that should be ok. 

 

[matthew-levi 14]

Love that warm glow of the tubes.

[JoaMat 1,171]

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. 

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

[matthew-levi 14]

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).

[matthew-levi 14]

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. 

[jamesmking 141]

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 October 7, 2020 by jamesmking

[JoaMat 1,171]

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.

[matthew-levi 14]

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. 

[matthew-levi 14]

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. 

 

[CrowDaddy 0]

Nice build, Matthew-Levi!

I'd like to do something similar; can you share your BOM?

Would you go with the same transformers again rather than a toroid?

Thanks!

[matthew-levi 14]

Hi CrowDaddy,

Thank you for the encouragements. I will send you the BOM I used. 

I think its a personal choice on whether to use an EI or toroid transformer. It's a matter of availability for me. I have since replaced the transformer with one that can also supply the 15-0-15V and removed the smaller LV transformer.