hi everyone,
disliking my yamaha amplifiers electronic phono stage (sounds ok, what you would expect, but the gain is very low) i was thinking of bypasing it internally and building a valve stage to replace it.
the only thing is that i am on a budget, so im looking to use 12ax7 long platers which i have here, then change to some genalex gold lions when i have the spare dosh as i have been told they are a huge step up from the average ax7 and are well worth it (opinions please!).
transformers are not a problem for me as i have loads of them and can make my own (have done before)
i just need some info on power supplies
i was looking at this one from audio innovations:
http://i31.photobucket.com/albums/c372/preludefan/ai500-ph.gif
it looks simple enough, however there is no supply schematic- i have built a few small amps before but am not sure about how to go about one for something so sensitive as a phono stage- my questions are:
what size capacitors to use in the dc heater stage to stop humming, and also there has been debate about weather to go ac or dc on this, i think dc is the way to go with a phono stage but again please advise
what is the best way to go for the hv supply rectifier ez81 or bridge diodes and also the right sized capacitors for these? i hear it will be more punchy with solid state?
also what voltage should i drive the ax7s to for pleanty of gain- 300v?
thanks in advance for your help!
oh and my player is a garrard sp25 mk3 with a magnetic cartridge
mike
disliking my yamaha amplifiers electronic phono stage (sounds ok, what you would expect, but the gain is very low) i was thinking of bypasing it internally and building a valve stage to replace it.
the only thing is that i am on a budget, so im looking to use 12ax7 long platers which i have here, then change to some genalex gold lions when i have the spare dosh as i have been told they are a huge step up from the average ax7 and are well worth it (opinions please!).
transformers are not a problem for me as i have loads of them and can make my own (have done before)
i just need some info on power supplies
i was looking at this one from audio innovations:
http://i31.photobucket.com/albums/c372/preludefan/ai500-ph.gif
it looks simple enough, however there is no supply schematic- i have built a few small amps before but am not sure about how to go about one for something so sensitive as a phono stage- my questions are:
what size capacitors to use in the dc heater stage to stop humming, and also there has been debate about weather to go ac or dc on this, i think dc is the way to go with a phono stage but again please advise
what is the best way to go for the hv supply rectifier ez81 or bridge diodes and also the right sized capacitors for these? i hear it will be more punchy with solid state?
also what voltage should i drive the ax7s to for pleanty of gain- 300v?
thanks in advance for your help!
oh and my player is a garrard sp25 mk3 with a magnetic cartridge
mike
300 V is fine, I would have two 330uf of capacitance with a 1K resistor between. use the bottom schematic, but change the plate resistors to 220K. as far as the heater circuit, just run a strait ac transformer (12V) and tie the center tap of the 12ax7's together and take a .1 uf poly cap (250V is fine) and go from the the common tap to ground.
JMO, steer clear of the "reissue" GECs in a phono preamp. THE current production 'X7 variant to use is the Sovtek 12AX7LPS, which is clean and quiet.
I've uploaded a tweaked version of the "classic" passive EQ RCA circuit. Several people have built it, with considerable success. As you found a lack of gain in the SS phono setup, substitute a 1 mA. constant current source (CCS) for the 2nd gain block's 100 KOhm load resistor. Any well filtered PSU of about 275 VDC is quite sufficient. Use an individual LR8 regulator in each channel to lock the rail at 250 VDC.
Don't forget a DC heater supply. Simple and highly effective is Greinacher ("full wave") doubling 6.3 VAC with 2X Schottky diodes and following the doubler cap. stack with a 7812 3 terminal regulator.
I've uploaded a tweaked version of the "classic" passive EQ RCA circuit. Several people have built it, with considerable success. As you found a lack of gain in the SS phono setup, substitute a 1 mA. constant current source (CCS) for the 2nd gain block's 100 KOhm load resistor. Any well filtered PSU of about 275 VDC is quite sufficient. Use an individual LR8 regulator in each channel to lock the rail at 250 VDC.
Don't forget a DC heater supply. Simple and highly effective is Greinacher ("full wave") doubling 6.3 VAC with 2X Schottky diodes and following the doubler cap. stack with a 7812 3 terminal regulator.
Attachments
finally i have time to get back to it....
i have built the 1989 version of this schematic, however there is almost NO bass.... and im unsure why... all of my component are within 10 percent of the values in the original schematic, which should make little difference, im using a stanton a600 cartirdge through this....does anyone think i need to inrease the capacitance from 51nf up?
thanks in advance
http://www.drtube.com/schematics/ai/ai500-ph.gif
i have built the 1989 version of this schematic, however there is almost NO bass.... and im unsure why... all of my component are within 10 percent of the values in the original schematic, which should make little difference, im using a stanton a600 cartirdge through this....does anyone think i need to inrease the capacitance from 51nf up?
thanks in advance
http://www.drtube.com/schematics/ai/ai500-ph.gif
What is the input impedance of the input on your amplifier that you have this connected to? For good bass response with that 12AX7A based design you are going to need something like an input impedance of 1M or so.. You could add a 6DJ8 based cathode follower to improve drive capability into low impedance loads.
This design is very similar to one of my very early designs - with some care this is a decent performer.
I don't see a 51nF cap anywhere in this design so I am puzzled by your comment as well as the comment about 10% tolerance parts, cumulatively this can result in several dB or worse of response error in the RIAA equalization. I use 1% resistors throughout, and tightly matched resistors and caps in the RIAA. (Better than 1%) This design is sensitive to the tolerance of the first plate resistor as well as the actual operating point of the tube. (Anything that effects rp will affect the response to some extent depending on design)
Take a look at SY's article "His Master's Noise" and my "Muscovite" thread for possible insights into the level of attention to detail needed to get a design right. Might want to look at the Russian 6S3P, 6S4P as potential fodder for a phono pre..
This design is very similar to one of my very early designs - with some care this is a decent performer.
I don't see a 51nF cap anywhere in this design so I am puzzled by your comment as well as the comment about 10% tolerance parts, cumulatively this can result in several dB or worse of response error in the RIAA equalization. I use 1% resistors throughout, and tightly matched resistors and caps in the RIAA. (Better than 1%) This design is sensitive to the tolerance of the first plate resistor as well as the actual operating point of the tube. (Anything that effects rp will affect the response to some extent depending on design)
Take a look at SY's article "His Master's Noise" and my "Muscovite" thread for possible insights into the level of attention to detail needed to get a design right. Might want to look at the Russian 6S3P, 6S4P as potential fodder for a phono pre..
Hi preludefan,
my opinion is that the RIAA circuit is not Ok.
here's a link how to calculate a passive RIAA network: Phono Stage Design, you may found a lot of similar calculation, if you look around on net.
The 1987 version is similar to one of its Audio Note's RIAA circuit, you should choose to Rg 1.82Meg then, the 1989/1991 version could be used if you put a cathode follower between the first ECC83 stage and RIAA network, plus change the 47k to a 37k resistor, htis is how I calculated it. Personally just did the some similar mistake, the difference is that I get the high's sound terrible, I'm on the way to correct it.
regards: kes
you will get no bass if you forget to bypass the cathode resistors. OR in the 1989 version of this circuit you must use a 47K Ohm resistior (yes, 47K) in the signal path or you will lose a lot of low frequency response...
Also, the 51pF input cap isn't really so necessary IMHO... the first 0.47uF coupling capacitor can bre reduced if you have any rumble problems though.
I hate to be the one to mention this, but 10% matching is not really good enough for phono - especially try to get the RIAA components to 2% or better (i.e. trim resistors by adding small resistors in series or very large ones in parallel).
Since you are on a budget I would suggest checking the first two things though (also check that your coupling capacitors are really 0.47uF).
regards
ian
Don't forget the 220k ohm resistor is in Parallel with the ecc83's ra (approx. 62.5k) + 47k ohm resistor - the results of the original designer is probably not so far off (I can't be bothered to check). But IMHO it's a nastly little trick that can easily be avoided by using larger correct value RIAA capacitors - which are as you note quite easy to calculate today.
The low bass is probably due to one of the three things I suggested above - namely cathode by-pass missing, too small coupling capacitor, or too small R1 (i.e. 47 ohm instead of 47K ohm).
Why do I think so? been there, done that... 😉
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Hi Kes - I just did some calcs and yes, this circuit doesn't come out correct 😉
If it were me, I might sell the long plate 12ax7's and get something different to work with. There are some recent phono designs with triode strapped pentodes using CCS or perhaps other designs based on Mu follower topology that I can highly recommend.
Ian
If it were me, I might sell the long plate 12ax7's and get something different to work with. There are some recent phono designs with triode strapped pentodes using CCS or perhaps other designs based on Mu follower topology that I can highly recommend.
Ian
This is what I get for RIAA values on this circuit. Of the many problems with this circuit, the size of R1 relative to the value of the 12ax7 ra is the greatest concern. It is not so easy to accurately measure ra, and it differs a lot among 12ax7 - especially as the valve ages, but also between different manufacturers.
Ideally R1 should be 10x the ra of the input stage, but this is simply not a realistic option when using 12ax7 for the input.
The other point is that you must take into account the miller capacitance of the next stage when calculating C2. I roughly put it at 150pf and added it to a somewhat standard available value of 0.0033uf to give 0.00345uf total.
Not sure if others would chose to do this differently.
Ideally R1 should be 10x the ra of the input stage, but this is simply not a realistic option when using 12ax7 for the input.
The other point is that you must take into account the miller capacitance of the next stage when calculating C2. I roughly put it at 150pf and added it to a somewhat standard available value of 0.0033uf to give 0.00345uf total.
Not sure if others would chose to do this differently.
Attachments
thanks for the info people, the compnents im using are not 10 percent tolerance, at worst they are 10 percent off the original valus. however i have used a 47k 1% resistor in the front of the signal path, i did stick to this as many phono preamp schematics use this, and it will be for a reason.
have tried bypassing cathode resistors, now it sounds worse.... might just order a bunch of new parts and start over....
have tried bypassing cathode resistors, now it sounds worse.... might just order a bunch of new parts and start over....
Make sure you get the polarity right when bypassing the cathode resistors. These capacitors should be little polarized electrolytic type. Elna Cerefine, Nichicon Muse, Panasonic or new old stock Philips..
For coupling capacitors you need polyester or polypropoline non-polar film capacitors. For the little RIAA caps traditionally people use polystyrene (be careful when soldering them since they do not respond well to heat) but silver mica are also very good.
If you have poor sound then it helps to describe it. Do you have hum? This can come from a variety of sources. It helps to use DC filaments to reduce hum, but in many cases hum comes from not twisting the incoming signal wires, or not shielding the amplification circuit.... Another tip I can give you is that the grid stopper resistors should be as close to the pins as possible to be effective at reducing noise from things such as RF hash.
Ian
For coupling capacitors you need polyester or polypropoline non-polar film capacitors. For the little RIAA caps traditionally people use polystyrene (be careful when soldering them since they do not respond well to heat) but silver mica are also very good.
If you have poor sound then it helps to describe it. Do you have hum? This can come from a variety of sources. It helps to use DC filaments to reduce hum, but in many cases hum comes from not twisting the incoming signal wires, or not shielding the amplification circuit.... Another tip I can give you is that the grid stopper resistors should be as close to the pins as possible to be effective at reducing noise from things such as RF hash.
Ian
You might consider a 1N4148 diode for cathode bias in the first stage - this can work surprisingly well compared to a cheap electrolytic and resistor. Note that this is actually fixed bias, and the dynamic impedance of the diode is consistent across the audio frequency range and on the order of 10 ohms or so..
Ten % away from the recommended values in the RIAA EQ network will cause egregious response errors, you need to be within a couple of % for decent conformity to the curve. I generally try to go tighter than this and have achieved accuracy of +/-0.1dB with some effort.
What is the input impedance of the amplifier input the phono stage is connected to?
Ten % away from the recommended values in the RIAA EQ network will cause egregious response errors, you need to be within a couple of % for decent conformity to the curve. I generally try to go tighter than this and have achieved accuracy of +/-0.1dB with some effort.
What is the input impedance of the amplifier input the phono stage is connected to?
Preludefan,
If you have not read this thread, it may offer some ideas..
http://www.diyaudio.com/forums/tubes-valves/213241-want-help-rca-tube-manual-riaa-phono-stage.html
If you have not read this thread, it may offer some ideas..
http://www.diyaudio.com/forums/tubes-valves/213241-want-help-rca-tube-manual-riaa-phono-stage.html
thank you all for your help so far. i have got a new set of components and i will try to assemble it tonight and let you know how things go
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