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Old 26th June 2019, 10:48 PM   #1
Cifte12AU7 is offline Cifte12AU7  United States
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Default Has anyone disabled or bypassed the volume attenuator in the TU 8600?

Has anyone successfully bipassed the volume pot entirely in the Elekit TU8600 amp? Would there be sonic gain from running the amp without the attenuator and using a preamp? I have the need for digital and analog single ended inputs and the Elekit has only one set of inputs. I would ideally like to use the unit as an amplifier only. Currently I either switch inputs manually or use a preamp and keep the Elekit volume at 12 o'clock, adjusting volume from the preamp.
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Old 26th June 2019, 11:47 PM   #2
Tromperie is offline Tromperie  Australia
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What is "sonic gain"?
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Old 27th June 2019, 09:55 AM   #3
paczeltf is offline paczeltf  Hungary
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Quote:
Originally Posted by Cifte12AU7 View Post
Has anyone successfully bipassed the volume pot entirely in the Elekit TU8600 amp?(...) keep the Elekit volume at 12 o'clock, adjusting volume from the preamp.
Why do you keep the Elekit volume at 12 o'clock if you want to bypass the pot entirely anyway? It is not a good idea as a logarithmic potmeter drops almost 90% of the input signal in the middle position so you may end up overloading the output stage of your preamp at higher volumes.

Here is a similar thread:

TU8500 + TU8600 - volume position

If I were in the same situation then I would use a passive preamp and a separate RIAA phono unit.

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Originally Posted by Tromperie View Post
What is "sonic gain"?
I am not a native English speaker but to me it means something like "sonic benefit" or simply "better sound".
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Old 27th June 2019, 01:09 PM   #4
Cifte12AU7 is offline Cifte12AU7  United States
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Interesting. So do you think I should turn it to it’s maximum setting?

Yes, sonic gain was meant to be better sound.
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Old 27th June 2019, 02:19 PM   #5
paczeltf is offline paczeltf  Hungary
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Quote:
Originally Posted by Cifte12AU7 View Post
So do you think I should turn it to it’s maximum setting?
Yes, at least for the following reasons

a) this is the position that affects the signal quality the least by far
b) the volume attenuator's position has impact on the bandwidth
c) you will get the same volume levels if you bypass the potmeter, which is what you want to do

The only exception to this rule would be a noisy input stage in the amp before the volume potmeter, but the TU-8600 does not have anything like that
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Old 27th June 2019, 03:57 PM   #6
Max Headroom is offline Max Headroom  Australia
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Quote:
Originally Posted by Cifte12AU7 View Post
Has anyone successfully bipassed the volume pot entirely in the Elekit TU8600 amp? Would there be sonic gain from running the amp without the attenuator and using a preamp? I have the need for digital and analog single ended inputs and the Elekit has only one set of inputs. I would ideally like to use the unit as an amplifier only. Currently I either switch inputs manually or use a preamp and keep the Elekit volume at 12 o'clock, adjusting volume from the preamp.
Any stages upstream are certain to degrade signal wrt direct connection to TU 8600.
My choice would be to add sockets and a DPDT switch to the rear panel of the TU 8600 and modify the pcb connections....cut two tracks (L&R) and add 6 wires (take care of wire directions).
Running preamp upstream adds extra cables and preamp noise/distortion/colouration which may be subjectively pleasing, but ime direct connection to power amp is best.
If you decide to run the preamp you could replace the amp volume pot with fixed resistors to set fixed gain and you can experiment with resistor types (MF, CF etc) and this will set a 'tone' to the amp that is likely preferable to cheap generic carbon volume pot and would also help in 'perfect' channel matching.




Dan.

Last edited by Max Headroom; 27th June 2019 at 04:03 PM.
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Old 27th June 2019, 06:13 PM   #7
planet10 is offline planet10  Canada
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Has anyone disabled or bypassed the volume attenuator in the TU 8600?
I am usually adding a volume (gain) control to my poweramps so that they can easily be used with PLLXO when i biamp. If you are using the TU-8600 as a power amp, turn the volume up all the way and it will effectively be out of the circuit (but for the shunt series R — often used as the gtid resistor for the following tube.

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Old 28th June 2019, 02:55 AM   #8
Max Headroom is offline Max Headroom  Australia
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Turning the TU 8600 volume to max will degrade system signal/noise, this is why I suggested replacement with fixed resistors or extra sockets/switch.


Dan.
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Old 28th June 2019, 11:27 AM   #9
paczeltf is offline paczeltf  Hungary
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Quote:
Originally Posted by Max Headroom View Post
Turning the TU 8600 volume to max will degrade system signal/noise
Why? Is there any active stage before the volume pot? I do not have the schematics of the TU-8600 but I have these for 3 other recent Elekit amps. All of them connect the first stage of the amp to the pot, which receives the signal from the input directly. I doubt the 8600 would be any different.

From a signal perspective, setting the volume to max in such cases connects the input to the first stage of the amp directly and also to the ground via an 50k Ohm resistor. This is how power amps are usually wired, the only difference is their somewhat lower sensitivity compared to the Elekit models.

Adding second input and a toggle switch is fine as long as you are willing to make irreversible changes to the casework and mess up the PCB and have the right tools, and you also accept the inconvenience of a having to use the input switch on the back.
I personally prefer a passive preamp.

Ferenc

Last edited by paczeltf; 28th June 2019 at 11:48 AM.
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Old 28th June 2019, 03:42 PM   #10
Max Headroom is offline Max Headroom  Australia
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I am assuming that the amplifier is setup to provide rated output with standard level input and volume control set at 12:00 with which is -20dB attenuation as per normal practice.

Amp vol control set to 100% therefore causes preamp needs to run at -20dB operating level and this causes loss of system SNR of 20dB at max system output, and worse at lower level system output.

Volume pot can be replaced with fixed resistors in 9:1 ratio to give same sensitivity as 50% volume control and discrete resistors may sound better than carbon vol control.


Dan.
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