Pre3 voltage problems

[bearded fiddler]

Thanks all for your kind words.

First findings:

- shunting the volume pot is absolutely necessary; even with 10.5 dB attenuation I hardly have a usable volume control range with the WAD 300B, which is of course very sensitive.

- hum on the loudspeaker terminals has gone up to 11mV on the right channel and 10mV on the left.

Better valves? I didn't go for the General Electric valve upgrade because I think they might be a bit lively to my taste.

Carbon resistors instead of metals for the shunt pot mod (I used Takman metals)? Or Shinkoh Tantalum (I used those in the "Phonodude" phono stage I built a couple of years ago and like them very much).

I'll let the Pre3 burn in nicely for a month or so before attempting any mods.

At this stage already the sound is very dynamic and realistic, and maybe a tad lively, even with single resistor cathode bias on the 300B. We'll see. The sound isn't harsh at all though: I stayed away from silver plated signal wire to avoid this. Most people won't agree, but that's the subjective sonic effect of this wire for me.

A final remark: I would have liked a switch on the PSU; now the Phono3S will be on all the time, wearing out its valves

Jan

[pre65]

If you switch phono3 off the pre3 HT will rise.

[Richard]

Originally Posted by bearded fiddler

Hi Richard,

A totally unrelated question: I did the shunt pot volume control mod: Alps 20K + 47K resistor. Does this arrangement seem ok to you or are the resistor leads going to pick up interference?

Jan

Hi Jan,

Missed this in all the enthusiasm to get the thing working! Yes that looks neat, very similar to how I did mine and there is no interference problem (do make sure the case metalwork and lid are all tied to signal ground if using standard 4 core umbilical or mains earth if using 5 core umbilical) and they will make a faraday shield.

See how you go with the volume range, mine was all in the first half but I didn't find it a problem. If you want more attenuation the 47K can be increased.

The other thing is to try moving your units around relative to each other. I found a main TX on a lead for the TT causede hum if directly underneath Pre3. It is not a big problem but simply lift/move things whilst switched on at playing volume to see if there are any issues.

Don't worry about Phono3 valves, they pass 1mA and will probably last 20 years or more of normal use.

[bob orbell]

Hi Jan, been following your post's, with ref to the hum voltage rising, is your output from the transformers in pre 3 (floating) or have you put the zero side to earth, if you have done the latter this can give a hum problem, just a thought .BOB

[bearded fiddler]

Hi Bob,

Could you be a little more specific, I'm not very technical. What is the zero side? Is it the CT connection? You'll have to spell it out for me

The Pre3 has no signal earth lift and the WAD300B does. Could that be a problem?

Jan

[bob orbell]

Jan, as standard, the pre 3's output's DO NOT connect to earth or the chassis, this is a FLOATING output, if you have followed the assembly to the instructions there will be NO conection from the output phono's to the earth post, some pre 3's I have seen have a conection from the phono earth to the chassis, this is incorrect and can be a problem with some power amp's. Hope this is a little better to understand BOB

[Richard]

Originally Posted by bearded fiddler

Hi Bob,

Could you be a little more specific, I'm not very technical. What is the zero side? Is it the CT connection? You'll have to spell it out for me

The Pre3 has no signal earth lift and the WAD300B does. Could that be a problem?

Jan

Jan,

If you've wired the outputs as per the manual the output will be floating. Grey (signal) will be on 7 and black (ground) will be on CT. That is correct.

Pre3 does have its signal ground lifted by 100R inside PSU. Pre3 signal ground is connected to the case and the whole is then connected to PSU signal ground via the umbilical yellow/green wire.

Check this by connecting just PSU and Pre3 with their umbilical (no mains or power amp or source connected). Put a probe on PSU and Pre3 case earth posts and you should find 100R. Put a probe on the outer shell of the Pre3 output rca and the other probe on either PSU or Pre3 earth posts and you should get no reading (no connection).

Bear this in mind when connecting any case earth posts to power amp or source earths;
1) PSU case earth post is at mains earth
2) Phono3 and Pre3 case earth posts are at signal ground lifted 100R from mains earth

This can be useful or can cause unintentional problems as using the wrong post may lose the preamp earth lift.

[bearded fiddler]

Thanks Bob and Richard. It's clear now.

I guess this is the explanation:

"Another advantage is due to the interwinding screens used in this advanced design. Effectively the primary and secondary windings are isolated from each other allowing us to break the ground connection between preamp and power amp thus avoiding the possibility of hum loops." (from the product description).

Jan

[John Caswell]

Hi Jan et al,
What you say is quite correct, however the interconnect between Preamp and main amp must have the screen grounded at some point, be it at the Pre end or at the amp end, but not both, although this does not always follow as it will depend very much on other interconnects and house wiring. Earth loops can be the very devil to sort out sometimes.
The main thing to bear in mind is that although the Pre Optx secondary has been wound primarily as a centre tapped winding for balanced cabling there is no reason why it cannot be used as a single ended winding.
There are 3 secondary output terminals on the tx "7", "18" and "CT". For a balanced circuit the screen of the cable would be connected to "CT" with +ve phase to "7" and -ve phase to "18".
For unbalanced circuits the screen can be connected to either "CT" or "7" and the output to "18". This will give you to have an overall gain of X6 (CT-18) or X12 (7-18).
It seems generally that most people require a gain of 0 to X2, and this can be arranged by "throwing away" a bit of gain by using the shunt volume control method.
For instance if your volume control is a 50K pot and the shunt resistor is 50K (51K actual) then at the junction of the shunt resistor and volume control you will have half the signal that is at the input end of the shunt resistor.
Juggling the value of this shunt resistor gives you a fair amount of control over how much gain there is throughout the preamp.
Hope this clarifies (confuses!) the situation.

John

[Greg.]

Originally Posted by John Caswell

It seems generally that most people require a gain of 0 to X2.John

Absolutely!! I'm still waiting for a satisfactory reason as to why WAD/WD have historically persisted with x6 or x12 or thereabouts gain

An explaination would be fantastic!