Author Topic: Ideas for MM valve phonostage  (Read 22921 times)

Offline fredeb

Re: Ideas for MM valve phonostage
« Reply #225 on: January 14, 2020, 01:13:56 PM »
A quick update: I disconnected my DC supply which I think was nearly overheating.  I tried simply connecting my AC 6.3V supply (with resistor 'centre tap') from the same ex-Decca amp transformer, and have got almost hum-free operation.
Google "filament humpot" or humbucker . I think it's a 50 ohm pot with a 22 ohm resistor on either side going to both 6.3V AC taps , and the centre of the pot going to 1/5 HT.  Turn the pot to tune into ideal low hum condition . I think Merlin has similar in the phono instructions .


Edit : it's called a humdinger
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline fredeb

Re: Ideas for MM valve phonostage
« Reply #226 on: January 14, 2020, 01:19:22 PM »
http://www.valvewizard.co.uk/heater.html

....further down the article , it's a 500 ohm pot , but I have seen others use similar values that I describe above .

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...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Qualityten

Re: Ideas for MM valve phonostage
« Reply #227 on: January 14, 2020, 01:27:54 PM »
My power transformer has an artificial centre tap installed (a slider across a resistive strip).  I've not measured the resistances.  Thanks for the reference to the very helpful Wizard's page.  I'll try the elevation at the weekend.

Offline Mars

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Re: Ideas for MM valve phonostage
« Reply #228 on: January 15, 2020, 01:06:19 PM »


I would try different values for C1. His recommended 10uF might be good, but I have found that electrolytic capacitors do not work as well as polyester/polyprop. I typically use 10nF (103 polyester).
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Offline handsome

Re: Ideas for MM valve phonostage
« Reply #229 on: January 15, 2020, 04:52:12 PM »
10nF is wayyyy too small - you really want as much filtering as possible at that point, otherwise you will be injecting HT noise into your all your valves. 10uF is a good value if you want to use film instead of electrolytic go for it (film will be physically much bigger though so rather inconvenient) but remember, there is no signal at the valve's heaters so fancy capacitors wont make any difference.

Offline Ampdog

Re: Ideas for MM valve phonostage
« Reply #230 on: January 16, 2020, 01:04:41 AM »
Few things (I have not gone over all the posts again, so kindly pardon repetition):

1. In pre-amps spikey d.c. on the heaters can easily cause more interference than a.c.  There is a fair capacitance between heaters and cathodes, apart from heater wiring going all over the place radiating, so keep fast rise-time occurrences out of there. (Those who simply rectify heater voltage for the sake of d.c. has little idea of the purpose of the exercise.)

2. On the matter of heater winding centre taps: Again the purpose is to 'balance out' 9,000 mVp a.c. heater effect on adjacent circuitry. But that only makes sense if such 'influences' are balanced. It makes far more sense to have a 'hum-dinger' pot there (how does 100 Hz hum 'ding'?) so as to be able to adjust for best cancellation of adverse effects, than simply accepting that such will be near 'balanced'.

3. Possibly already pointed out, but never leave a heater circuit just 'hanging in the air' (i.e. no reference to earth other than what internal capacitances may see to. That is exactly what will then happen - possibly causing over a.c.-voltage/heater-cathode break-through in some unfortunate valve. And if in your power transformer a heater winding may be adjacent to the 'live' side of the mains or h.t. winding(s) and spikes come along . . . go figure).

4.  Again may have been pointed out, but be careful of using heaters in series for a more convenient higher-voltage-lower-current d.c. heater supply. Heaters not specifically designed for series operation can draw significantly different currents, causing undesirable differences in cathode temperatures. (And yes - there are debates going on whether constant voltage/current/power is best for valve heaters; to the internet for those! As I recall 12V 0,15A heaters may be connected in series; been doing that myself. Perhaps a member can shake me out of my slumber. . .  )

5. Also already mentioned?:  Consider series F.E.T.s for h.t. voltage smoothing. Might well work out cheaper than umpteen bulky electrolytic caps, plus allowing for some h.t. regulation.   
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline vinyljan

Re: Ideas for MM valve phonostage
« Reply #231 on: January 16, 2020, 07:10:33 AM »
Few things (I have not gone over all the posts again, so kindly pardon repetition):

5. Also already mentioned?:  Consider series F.E.T.s for h.t. voltage smoothing. Might well work out cheaper than umpteen bulky electrolytic caps, plus allowing for some h.t. regulation.   

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A shot across the bow

Offline Qualityten

Re: Ideas for MM valve phonostage
« Reply #232 on: January 16, 2020, 09:58:13 AM »
 Any information on what U1 is?

Offline handsome

Re: Ideas for MM valve phonostage
« Reply #233 on: January 16, 2020, 10:55:24 AM »
It's a VTA HVR SILR of course!

It could be a LR8

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Offline vinyljan

Re: Ideas for MM valve phonostage
« Reply #234 on: January 16, 2020, 11:29:26 AM »
 :dop:

A shot across the bow

Online charles

Re: Ideas for MM valve phonostage
« Reply #235 on: January 16, 2020, 03:46:40 PM »
There are so many circuits. What about its specifications and test procedures.

Some advice, use a scope to check ripple, etc when building a power supply.

Offline fredeb

Re: Ideas for MM valve phonostage
« Reply #236 on: January 16, 2020, 07:36:35 PM »
Few things (I have not gone over all the posts again, so kindly pardon repetition):

1. In pre-amps spikey d.c. on the heaters can easily cause more interference than a.c.  There is a fair capacitance between heaters and cathodes, apart from heater wiring going all over the place radiating, so keep fast rise-time occurrences out of there. (Those who simply rectify heater voltage for the sake of d.c. has little idea of the purpose of the exercise.)

2. On the matter of heater winding centre taps: Again the purpose is to 'balance out' 9,000 mVp a.c. heater effect on adjacent circuitry. But that only makes sense if such 'influences' are balanced. It makes far more sense to have a 'hum-dinger' pot there (how does 100 Hz hum 'ding'?) so as to be able to adjust for best cancellation of adverse effects, than simply accepting that such will be near 'balanced'.


Hi Ampdog , once again , thanks for the advice .  :2thumbs:

I was wondering , concerning the two points above , if one could configure a low-pass filter at 30hz for DC ( calculated at 33hz ) with 0.47e resistor in series and 10 000uF cap in parallel with supply ?

Rise time is calculated at 0.03 seconds - still too fast ?

http://sim.okawa-denshi.jp/en/CRtool.php
« Last Edit: January 16, 2020, 07:40:45 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline handsome

Re: Ideas for MM valve phonostage
« Reply #237 on: January 16, 2020, 09:56:14 PM »
Heaters in preamplifiers especially high gain ones like phono stages or microphone preamps really should use DC. As Ampdog points out just rectifying and capacitor filtering a heater feed can create more problems - you go from fighting just 50Hz hum to high frequency hash from the rectification process. Rectifying and filtering 6.3V windings is unfortunately a compromise though it appears on many designs... Ideally heaters should use DC from a separate transformer to avoid HT rectification spikes coupling via interwinding capacitance into the heater winding, and be regulated. Regulation is cheap and effective - LM317T guarantees at least 60dB of noise suppression at ~100Hz. A 10uF capacitor across the lower voltage set resistor increases this by 20dB and even more can be had with a judiciously sized transformer and an RC filter before the regulator. Heater stand-off/bias feeds must also be well filtered - easier since the resistances involved are so high - and their voltage high enough to be well within the limits of the heater to cathode voltages specced for each valve. This is most pertinent when DC coupling, cathode followers, cascodes or any other series tube arrangement is used.

Offline Ampdog

Re: Ideas for MM valve phonostage
« Reply #238 on: January 17, 2020, 07:25:08 AM »
Fredeb,

What you suggest is rather theoretical, compared to what can one of several voltage regulators can achieve . . . .  One simply uses such and get matters done with as mentioned.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline MusicMan_ZA

Re: Ideas for MM valve phonostage
« Reply #239 on: January 18, 2020, 01:20:51 PM »
After taking a break for a few days, I have been listening to my BottleRocket extensively for the last two nights. So, after 10+ hours of playing time, I can only say: this thing sounds amazing! I really enjoy the sound, it is very quiet, and I will not be making any changes to it, at all. Next is just to have an aluminium faceplate made, and possible "engraved".



I love music - analog and digital. Even listening to the Blues makes me happy. During the day I coach and counsel, after hours I indulge in my second passion - listening to vinyl and restoring turntables. Some I keep, some I sell...