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Preamp PSU Design Help

Hey all,
I'm pretty new around here, been browsing for a while but this is my first post in this section.

I'm buildign a Line preamp useing 1x ECC88 in parralell per channel from a schematic i found on the internet. I have already sucesfully built a preamp useing just one half of an ECC88 per channel but i have a lot of problems with hum and i have decided to make a new one that is neater and try to get rid of the hum problems.

The new preamp i am building says it needs a supply of +250VDC.

I have a transformer i am hopeing to use for it (i have limited budget and i already have this transformer) that has secondary windings for heater supply and i believe a 375 - 0 - 375 HT winding (measured it with a big old analogue clamp meter and thats the voltage readings i got).

So basically i am wondering what the best way would be to go about makeing a decent PSU.

I was thinkign of useing full wave solid state rectification and then haveing 3 R/C filters - 330Ohm/330UF into 330Ohm/100UF into 330Ohm/47UF problem is i dont really know much about PSU design (even though i have a HND in EE) so i am open to sugestions and comments about what i plan. i havent ruled out useing a choke but i would need somethign quite small (trying to make it as small as possible) and also not too expencive (poor student).

Thanks in advance for your help,
Owen
 
I did search but i can't have done it very well as i missed that thread :ashamed:.

I'm Totally confused now, I reacon i need to measure the output voltages from the transformer with something else to be sure what they are, mabey my meter is out of calibration or something, as the capacitors that were connected to the transformer are all 450VDC cap's. infact the outputs from the transformer went straight into two diodes and into a 450VDC capacitor but if what i read in the other thread is right then 375 - 0 - 375 secondarries would mean that the DC voltage is way too high for 450VDC cap's? :S

Owen
 
For this desing, I used 285-0-285 sec. winding into AZ11 tube rect. You`ll get around 350V after rectifier. After that 47uF/3,3K/47uF/3,3K/47uF CRCRC will take you in 250V ball park. I actualy used one moore filter in front of CRCRC (CL) but woltage drop is only an aditional volt or two.
 

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Thats the same circuit i was going to try, how do you like it? I was pokign around today with a DMM on my old preamp i built which is from the same site i found that one on, it uses a single half of an ecc88 per channel and the power fransformer i am useing for that has a coupleof 0-6.3 secondaries, a 0-150 secondary and the only reason im not too keen on useing it is it has a 0-1200 secondary! but i measured it today and the 0-150 secondary is kicking out 180VAC when it is only driveing one ecc88, i'm thinkign of stripping that preamp now and useing the power transformer from that instead. Not really sure if i want to build that parallel design or if want to build an SRPP i have schematics for. the SRPP only needs 165V so would probably be better suited to the components i have here. Just need to work out which would sound best with my other gear now. For the next 2 years it will be running into a yamaha AX-590, connected to the main in (bypassing the internal preamp) source is a Sony CDP-711.
 
Despite all negative opinions about paraleling triode sections, ecc88 seems to work very god in this configuration. Actualy I was suprised by quality of it`s performance. Open, very dinamic, and linear sounding. It outperforms my trusty Electrocompaniet EC 4.6 in many aspekts. I had a chance to hear that circuit (it`s a J.Curcio "Daniel" preamp-line stage section) before building my. Original version is using stabilised PS, but crcrc filtered, tube rectified PS is working like a charm. I used a transformer from a old 1960 radio, rated at 280-0-280, 4V, and 3,15-0-3,15V. Perfekt and cheap solution.
 
i'll give it a go soon, i have some old valve radio's i bought for parts but i havent had a chance to check them out yet, dad says they arent anything special (antiques dealer) he bought them in a job lot at an auction because there was one in the lot he wanted and gave the rest to me. :) hopefully there will be a suitable transformer in one of those. I'm building an ECC88 SRPP linestage as i type just to see what it sounds like in comparison to the single triode ECC88 linestage i built before. then when i find a decent power transformer i can build a paralell one and see which one i prefer (have LOADS of NOS and ex equipment mullard ECC88's some british, some made in holland so plenty to experiment with :D )
Owen
 
finished my srpp linestage last night, im loveing it, so much better than the old one, a lot less hum, running AC heaters again without any hum problems, only fault i have with it is it has way too much gain, even running from the variable output on the CDP with the cdp control set to half it still has too much gain. but it sounds great and aslong as i run a pot on the preamps output set to about half it works great. How would i go about reduceing the gain? is it possible? i'm thinking mabey i should lower the B+ voltage a little, i'm running 180V, schematics say to use 165V but i found it sounds better at 180V. will running it at this higher B+ have any nasty side effects?
 
90 V. on the plate is "textbook" for the 6DJ8. 180 V. B+ looks GOOD to me for a 6DJ8 SRPP.

You can turn the excessive gain to your advantage. The Rp of the ECC88 is low, which makes it good for driving a trafo. The MagneQuest B7 trafo in the 5:1 ratio would reduce the O/P voltage by a factor of 5 and also reduce the O/P impedance by a factor of 25. The B7 also comes in a 3:1 "flavor".


While we are on the subject of "iron", you might want to look into a "tweak" developed by Australian DIYers. Choke load the SRPP .
 
hi, Just thought i'd post the schematic for the srpp i built, i found this one in an old post on diyaudio. only thing different i did to that schematic was use a 0.6uf capacitor instead of the 0.68uf one in the schematic (only had a 0.6 in my parts bin).

If i can source a suitable transformer in the UK i will definatley give it a try, sounds like its just what i'm looking for, will look into choke loading aswell and see what i turn up.

Overall im very happy with the preamp, bass response is deffinatley up on the last ecc88 pre i built, sound is very clear, soundstage is soo much better than running without the preamp and it seems to like all the music i have thrown at it so far which is good because my music taste is VERY varied, i like pretty much anything as long as im in the right mood at the time. :)

Owen
 

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PS help

hi,
i did not get it if you solved properly the PS problem, but if you
need a 250 vdc as +B the secondary voltage is far too high,
the most elegant way is to use a voltage regulator, a full schematics is available from ' Elector ' if you are interested i can look for it. The over gain problem is critical only when is a noisy
output, but if you like to reduce the gain just a simple voltage divider will do it at the main or selected input of the amp.
best regards
Williams
 
I have looked around on google and i cant find a uk distributor for that transformer, i found a similar unit made by sowter for £34.57 each
datasheet: HERE


they also have one that is 4:1 and designed for lowering the output level from cd players and stuff which is cheaper at £28.56 each.
datasheet: HERE

would either of these be suitable?
these will cost more than i have spent on the entire preamp so far but still not as bad as the otheres i had found before comeing across these, they were all between £80 and £350 each! which is kind of outside my budget, my budget will just about stretch to these sowter units unless i wait a couple of months which i dont think is going to happen, dont like leaving stuff half finished. :)
Owen
 
thanks for the reply williams.
I have given up on the paralell ecc88 preamp for the time being (untill i find a suitable transformer) i have built an SRPP preamp instead which i am very happy with, it's still in the "tweaking" stages at the moment though.

What are the advantages / disadvantages of a voltage divider on the input compared to a transformer on the output (or input for that matter)?
Owen


EDIT: just a thought but mabey i should make a new thread? this one is going a bit off topic now i think?.
 
Thread Revival

DoomPixie said:
The new preamp I am building says it needs a supply of +250VDC.
I would actually like to revive this thread...

I am making an RIAA/phono preamp. THIS is the thread

I am having a problem getting a clear answer on the subject of a PSU for my particular amp.

You see, I also need +250 V, and I don't know much about voltage regulation; in the old RCA manual the subject of filters is discussed. The choke-capacitor method of filtration is easy enough to understand- capacitors store voltage when it's at it's peak, release it when it's at zero.

I was going through my Thordarson trasformer manual, and found several power supply circuits; all of them from 1935, but the theory behind them is the same.

Would any of THESE circuits be suitable for my preamp (keep in mind that I would like to convert over to solid-state diodes)? Or is further voltage regulation required for less hum?

THIS is the transformer that I will be using.
 
A regulated B+ PSU is, IMO, a good thing in a phono preamp. The 500 VAC CT rectifier winding will yield approx 350 VDC if a cap. I/P filter is employed. That extra 100 V. will be very useful. A stack of a VR105 and a VR150 will take care of regulation. Don't worry about the extra 5 V., as it's WELL within tolerances. UF4007 SS diodes are inexpensive and relatively quiet. Hop over to Audio Asylum. Scan the Bottlehead Forum for RRSF. You will obtain info about a filter that cleans up the UF4007's switching noise.

A phono stage NEEDS DC on the tube heaters. The filament winding of the linked trafo is up to the job. Construct a "full wave" voltage doubler made from 2X Schottky diodes. Use BIG caps. in the doubler stack. The doubler feeds a 7812 3 terminal regulator.