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  #11  
Old 8th August 2019, 03:54 PM
MarkL MarkL is offline
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Default Re: WD Pre III CF

So, first problems. I was trying to work out how the pre2 cliffcon connections related to the different arrangement on the pre3.

So I tested the voltages from PSU2 to work out which was which.
For the 2 heater voltages I got 41V and 48V, the HT voltage was 128V instead of 100V. Great. How was it even working? I also checked the phono2 HT and that gave 343V(not 250V) Mains in is 254V today. All measurements done with 2 different meters for good measure.

Suggestions most welcome!
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  #12  
Old 8th August 2019, 05:12 PM
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Phil Y Phil Y is offline
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Default Re: WD Pre III CF

Hello Mark.
The heaters are raised to protect the CF valve. Measure on the 2 heater wires, not to 0V, although it looks close (7 volt difference)

The HT will be high with no load as it is not regulated. Measure with it connected to the amp or a dummy load.

Phil.
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  #13  
Old 8th August 2019, 05:38 PM
bob orbell bob orbell is offline
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Default Re: WD Pre III CF

Does the CF not need 200-250 volts ?, I know the Pre II used 100 volts and was regulated by a Zener diode, my CF runs at 250 volts, regulated. Bob
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  #14  
Old 8th August 2019, 05:47 PM
bob orbell bob orbell is offline
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Default Re: WD Pre III CF

Looking into this a little further, I do think you will need to mod the PSU II as 100 volts is not correct, the Pre III CF is made to work with the Pre III which gives 250 volts, so a bit of resistor change will be in order, but the elevated heater supply will be OK. Bob
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  #15  
Old 8th August 2019, 06:09 PM
MarkL MarkL is offline
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Default Re: WD Pre III CF

Thanks Phil. 6.3V!
Helps when you know what your doing.

Thanks Bob, I have some mods to make on the PSU, but I needed to know it was working properly first. I think I have to swap a resistor and take out a diode.

I can crack on now and keep you posted!
cheers
Mark
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  #16  
Old 18th August 2019, 07:12 PM
MarkL MarkL is offline
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Default Re: WD Pre III CF

So, pre3 CF is finished. I have configured it to constant current sink mode, and it sounds, well, nice for want of a better word.

On first connection the top end was good straight off, midrange was a little thin, bass was epic. I have always found new stuff always sounds below potential prior to burn-in, and this is no exception.

20 hours in the midrange is lovely, detailed, very easy to listen to and not too "hifi" The bass is so good I keep looking for tracks with lots of it so I can see what it will do.

If I were to do the same again, I would consider paying the extra for the full kit. It took me a while to work out the best way to adapt one kit to the other. Not difficult it just took a while.

A practical issue with PSU2. I changed R4 for a 15K resistor and removed the zener diode. I now get the same voltage at the anodes for Pre3 and Phono2, 343V unloaded. PSU3 would give 380V - 420V.

So all my other voltages are down. I get 140V instead of 200V at the anodes of the valves, and 63V instead of 105V at the cathodes.

I don't know how this affects the circuit, but I couldn't get the 2 diodes to stay lit when I adjusted the trimpots to 0.84V. Best I could get was 0.79V

Sounds great, so is the above a non-issue?
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  #17  
Old 26th August 2019, 01:56 PM
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petercom petercom is offline
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Default Re: WD Pre III CF

Quote:
Originally Posted by bikerhifinut View Post
My take on it is that any properly designed CF circuit will be neutral sounding and simply just do its job of buffering an input without drama.

A.
I wish that was the case! Most CFs are variable in their characteristics according to the impedance they are driving.

This was the raison d'etre of the Pre3 original design, to be able to use the preamp at a distance from the power amplifier, using the output stage via transformers, to drive long leads without compromise.

If you're using the Pre3 CF, or any CF output stage, I'd put it close to the power amp and use short leads with well-balanced impedance (not prone to high capacitance). Then it should work as intended.

BTW, I demonstrated the Pre3 with output transformers against a very well known, and highly regarded, passive transformer-coupled preamp. You should have seen the look of complete astonishment on the 'highly regarded' passive preamp owner's face when we switched to the Pre3. He couldn't understand how his expensive, passive unit was inferior to an active design. I can.
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