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Need power supply help for Precision Fidelity C8!
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Author:  CrazzyAbtTubes [ 19 Aug 2009, 13:42 ]
Post subject:  Need power supply help for Precision Fidelity C8!

Now that I have the phono stage done, I need to make the power supply to give it a proper test on high voltage. I found this regulated HV power supply a while back that uses a single TIP50 and a zener diode to set the voltage, and has a voltage range of 250-350V.... the only problem is that I can't find the site that had it! :bawling:

I have done many modifications to it to suit the requirements of the preamp, about 200V to keep things cool for now and to stay on the safe side of the capacitor ratings ( the originals were ither 200V or 300V meaning I had to upgrade some of them).

I have decided to use a 5Y3 rather than diodes (just because I can, and they look cool ;) ) and I want to have a HV relay that will enable the HV after 30 seconds or so, and the voltage I want is also lower than what the schematic says :confused: I'm not sure if this will be a problem or not there are a couple of resistors.... then the zener.... and some random capacitors, and no I have never tried this power supply before and really need some help to understand how it works because I can't find the info for it!!! :x

So all I can do is take some pics of what I have along with the schematic....

Author:  tombethe [ 19 Aug 2009, 17:08 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Hi Alan,

this PS is working like a "capacitance multiplier". The IC (Transistor) "amplifies" the capacitor that is connected below the 10K / 5W resistor. The whole thing works this way:
5Y3 rectifies, the CLC is the first filter, that provides a basic ripple smoothing (yep - i know that you know that). After this CLC, the cap multiplier starts to work. The decreased ripple current (coming from the CLC) has to pass the source / drain from the TIP. The TIP smoothes the ripple current again, by re-feeding an ammount of the output voltage through the gate and by switching the current from the source to drain. The TIP amplifies the current by the "β factor" that should be found on the datasheet. The 10K / 5W resistor sets the bias for the gate + loads the cap. The cap unloads the ripple reduced current through the gate. The Zener just is a protection for the limited "gate to source" voltage. The cap after the TIP is for buffering the smoothed output voltage. I wrote source, drain and gate, because i didn't surely know, if the TIP pins are also called collector, emitter and base in the states. Collector=source, emitter=drain, base=gate.

Also see: and

One of the best cap multipliers can be found on Thorsten Loesch's site (Thunderstone Audio) : (Scroll down to: Zero-loop-Feedback Series Regulator with output decoupling )

The bias resistor MUST be calculated to stay below the max. allowed gate to source voltage, if you don't want to use a pot.

A cap multiplier works fine! I use it in my Szekeres and a transistor amp. Also in a tube amp (that still isn't ready)...

I hope that helped ;-)

Author:  CrazzyAbtTubes [ 19 Aug 2009, 20:58 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Thanks tombethe for the working description, that actually helps quite a bit, I was pretty sure that the 10K resistor would have tobe changed because I have seen other similar designs, but what value should I change it to is now the question, although I would probably be better off using the information you provided and calculate the value for myself.

The main reason for me using this design is that the preamp allready used one very similar to this one, I ditched the old power supply because some one had obviously tampered with it and the poor TIP50 regulator was running at 85 digrees C !!!! :hot:

But I would like to keep the use of the TIP50 as not to change the original sound too much, this is also a strange breed of Hybrid, so it will have an entirely diferent sound from that of a straight tube preamp. When it is done I am going to provide all the info for someone to build it straight from scrach using common parts that can be salvaged from whatever junk one may have. ;)

Any way back to the questions.... In the original schematic there is a limiting resistor after the rectifier, I decided to use an inductor instead that hopefully will do the same job but also filter noise as well (the phono stage is very sensative to noise, also why I am sicking with full regulation). But I also have a relay before that and would like to know what kind of inrush current I would get when it finally enables the high voltage? :?

Other than that I should be good, but let me know of anything that should be changed.

Author:  tombethe [ 20 Aug 2009, 05:13 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Hi Alan.

85 degree C is a lot. Imho, the transistor is getting warm, but not that much. I think about 45 - 55 deg. C should be normal.

In the original schematic there is a limiting resistor after the rectifier, I decided to use an inductor instead that hopefully will do the same job but also filter noise as well
The inductor nearly does the same job. The resistor you meant is a part of the ripple / hum filter (CRC filter) after the rectifier. The resistor value is calculated to build a low pass filter, within the loading and filter cap, that reduces the hum. The inductor and its AC-resistance does the same, in an other way. Using the inductor shouldn't be a problem, but i'ld take a look on the datasheet for the max. allowed loading cap for the 5Y3, just to be on the safe side for the 5Y3. Imho - rectifier tubes don't like high backflow currents from the 1st. loading cap... The CLC-filter should work as well as the CRC-filter. The only big difference between CRC and CLC is, that CRC filters dissipate some volts through the resistor.

But I also have a relay before that and would like to know what kind of inrush current I would get when it finally enables the high voltage?
Imho - that is not a problem. On your schematic, you planned to integrate the relay before the loading cap (CLC-filter). So you'll have a integrated "soft start" function. The caps and the "cap multiplier" will need a while for charging / discharging... I'ld suggest to put a bleeder resistor after the relay (2,2k 2W in parallel), to ensure that the caps are unloaded in a minute. (Turn off / on protection).

Now i think you want to have some values for the bias resistor and the zener?
Now - there are three ways to do it. The max. allowed emitter to base voltage is 5 Volt. You can engage the resistor to set the bias and use a zener for protection - or you can engage the zener to regulate the base voltage, and use the resistor as zener protection - or take the first two ways and add a pot to ground, to adjust the base for an adjustable output...

The resistor is calculated by using the transformer current and the resistors voltage drop to get about 5V (zener as protection used).
Or calculate the voltage drop to get the max. allowed supply voltage for the zener (then the zener regulates the base voltage)
In both cases, you'll have the full PS voltage at the output! The TIP is completely open and not regulated then.
One step further: Add a pot to the resistor and you can adjust the output voltage by regulating the base voltage. EZ ;-)
I'ld choose a 2W - 5W zener for the protection. A ZD47 (4,4V - 4,7V zener voltage) or a ZD51 (4,8V to 5,1V zener voltage) should be choosen, where i'd prefer the ZD47.

And i'ld suggest to take a look on Thorsten Loesch's article (posted before). Thorsten really knows what he does ;-) He builds the output regulated supply for his preamps (also for the RIAA phono preamps).

I hope that helped further..

Author:  CrazzyAbtTubes [ 24 Aug 2009, 13:47 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Ok, so I am drawing the schematic now and would like to clairify exactly were I would put the pot to vairy the voltage? Do I put it from base to ground? or sub the 10K 5W for the pot?

Also about the zener, would a 4.3V 1W be OK? The only 5 watt zener is the one from the original preamp supply which is 200V, and what would be a good value for the pot?

I would work out the values myself but don't understand the circuit enough to do so and don't have the time ither. :worried:

Author:  tombethe [ 24 Aug 2009, 16:25 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Hi Alan,

i did a little schematic...

Just calculate the resistor R1 to get the 5V to the transistors base. The pot goes from the R1 to ground to adjust the transistors base voltage. (think of the dividing within R1). If the voltage is decreased through the pot, the transistor decreases the output voltage by closing the base more and more. The Zener should be a 4,7 ZD where 2W should be enough. The ZD only ensures, that the transistors base voltage doesn't exceed the transistors emitter to base limitation of 5 Volts and could be seen as an protection...

Ican't see which zener was placed in the schematic... 1N538 and somewhat... This should be a 140 - 160V zener voltage and surely blows up the transistor!

The mainpart on this circuit is the R1, the pot and the cap C2. They form a low pass filter and the transistor amplifies the supply current by the transistors "beta" factor...

The pot can be a 100K. It is set to ground so you have 50K at half the turn. I don't know the transformers current...So I'd start with i.e. 10k (the sure side) for the R1 to get the base voltage down to 5V. The pot lowers the base voltage again by dividing the 5V to decrease the output voltage...

Author:  CrazzyAbtTubes [ 24 Aug 2009, 18:30 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Yea, I really wish I could find the site with this schematic, if you go back to the text it explains how it works and what values do what, sadly the link does not work so I am stuck, and to get a 4.7 volt 2 watt zener would take another weekly order, and school goes back in a week! :bawling:

Any ways thanks for the schematic, but I do have one last rediculouse question.... if a zener does it's job keeping the base at 5V, than couldn't a 7805 be used instead some how? it could easily be set to in the range of 4.7 volts if necessary, but by the looks of it I am stuck with a rare zener value unless I go for one of the other earlier designs you suggested, which I may be better off.

I really wish I had more time, I wonder if the tube regulated HV supply would be more successful, I have a schematic that uses a 6CG7/6FQ7 as the regulator and a ECC83/12AX7 for voltage control, but I don't know if it has the output current capable for this preamp.

Author:  tombethe [ 25 Aug 2009, 11:31 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

If it's not so easy to get the ZD 4,7 you also could leave off the zener... The zener doesn't set the transistors base to 5V and it doesn't make a sense to replace it with an pos. inline regulator. With using an inline regulator you don't get the low-pass filter to the base.
And an 7805 stes the base voltage to 5V fixed. If you have no voltage at the emitter (no signal to amplify = curent flow = 0) the transistor really doesn't feel good. The emitter current always has to be higher than the base current. For this, the base must have the resistor R1. The ZD is the insurance, that the transistors base voltage doesn't reach the max. allowed 5Volt limit. OK - you could also see the whole thing as a tube. The collector is the plate, the emitter is the cathode and the base acts like the grid. The base must be biased to get a quiescence current on the collector - emitter rail. (~0,6V = lowest current - 5V = max. current)
Like on the tube: Positive base current (voltage) = emitter current flow (voltage), decreased base current (voltage) = decreased emitter current (voltage).
Leave off the zener, but be sure, that the voltage to the base doesn't exceed 5V. As said - the pot is to decrease the voltage again - and to decrease the base voltage - to adjust the supply output voltage.

If you have no time and also no time for checking out the R1 value start with a higher available value like 100K and just measure the PS output and the voltage to base. Once you got the base voltage you could replace the R1 with i.e. 80K and measure it again. By calculating the voltage difference between 100K and 80K, you really should be able to calculate the finally needed resistor value!

Author:  CrazzyAbtTubes [ 25 Aug 2009, 12:16 ]
Post subject:  Precision Fidelity C8 Power Supply Schematic

I did a bit more searching and got lucky. :mrgreen: I found the power supply schematic for the Precision fidelity C4 preamp and it is a similar design to the C8 so I was able to draw up a schematic and this would be even easier to make because I have all of the parts on hand.
Precision Fidelity C-8 Power HV supply(small).jpg

Author:  tombethe [ 25 Aug 2009, 13:53 ]
Post subject:  Re: Need power supply help for Precision Fidelity C8!

Congrats Alan!

Ok - this circuit also has lower hum / ripple. It should be very clean as a darlington config is used.

Have fun ;-)

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