• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Simple Valve Preamplifier

Hi Guys,

All of my equipment is solid state and I've always wanted to try out valves for their supposed "warmth" etc. I've never heard one, so I wanted to try building one to see how they go.

I've done a search of this forum and a simple one that I've found is the Silicon Chip 12AX7 Valve Preamplifier. I like the simplicity of this as it only needs 2 tubes for a stereo setup. I know most of you guys don't like this design/valve, but it seems to be the easiest to build from what I've found so far.

URL: http://www.google.com.au/url?sa=U&s...onchip.com.au/cms/A_30804/article.html&e=9707

So do you think this would make a good beginners valve preamp and give a realistic example of how valves sound?

If not, do you guys have other schematics etc that I could use? In particular the power supply that could possibly be used with the silicon chip version as that one seems complex in that you have to wind your own transformer, and I've never done that before.

Thanks.
 
G'day Mangrovejack,
Good to see another Aussie getting into valve stuff.

See this thread from another Aussie asking about the same "preamp".

http://www.diyaudio.com/forums/showthread.php?s=&threadid=54614

About the ONLY thing it has going for it is its price. Its dirt cheap - to match its performance. You can do heaps better. Use the search function to look for "Simple Line Stage" or "6SN7 Line Stage" and similar. Lots of good circuits here and 98% of them will give better results than the "Silicon Chip" magazine circuit.

You could, however, use the "Silicon Chip" Board/Article to do your own experimenting and get something which will add that bit of valve warmth to your solid state gear. Start by chucking the 12AX7 and use 12AT7 or 12AU7. I did'nt read thru' the entire thread shown above BUT I suggest you do so to see what others had to say/suggest.
I had a copy of that issue of the magazine so was already familiar with the circuit and not greatly impressed but as stated above if you use it as an experimenters board rather than a final project then may be worth the low cost.

Cheers,
Ian
 
Hi,

That is not a bad sounding preamp and it beats most solid state preamps. Even though it is probably not a very good sounding valve preamp by comparison, used in my system (in which the preamp is the weakiest link) the entire system still beats most commercial systems priced 5 digit and above. I comparied it on my system with Audio Research LS2 or LS8 valve preamp (can't remember the exact model) that used to be retailed at $4000 or something, and the sound quality matches about 75%. I have built one and used it for 6 months. I am now in the process of building a better (or supposed to be much better, even comparing it with Audio Research) one with 12B4A, EI trany, choke input, ccs fed shunt reg, etc. Much more complicated though. I will post the result in a few weeks time when I finish it.

If you have good experience building electronics and can understand and model the PSU, etc then you may look for a better preamp, otherwise that one may get you started. If you want to save money on that preamp you can buy mine for half of the price you would pay from Jaycar. That will be well under $100 and it is working. It will give you fun but I don't think you will keep it for a very long time. The problem with this kit is that the switch mode PSU is noisy and you have to be nearly one metre away not to hear it when your room is quiet.

Regards,
Bill
 
Mangrovejack,
Just looked at your details and I see that you are a software engineer. As a hardware engineer myself I don't want to discourage any "softy" from understanding the hardware better - far less demarkation disputes that way. (How many Software Engineers does it take to change a light bulb? - none, its a hardware problem.)

My advice, for what its worth, is that if you seriously want to experience that "valve sound" then the preamp/line stage is NOT the place to start experimenting. You really need a valve POWER AMP.

You may like to look at some dirt cheap but well performing Chinese Amps available from people like the "audioguy" on-line shops out of Hongkong or similar.

I have a Chinese 845SET "Music Angel"which with a bit of tube rolling and a few upgraded components is very respectable. These amps are being brought into Oz by Scott Thomson at Rarefaction in Melborne BUT its cheaper to bring it in yourself. Scott auditioned the full "Music Angel" range and was most impressed with the 845 SET and the EL34 Push Pull which is half the price of the SET. You might want to check with Scott to see what he has in stock - you'll pay a bit more BUT you don't have any of the hassles associated with dealing with on-line companies, customs clearance etc. and you get local support, thats all worth a few dollars. .

Cheers,
Ian
 
Ian,

I fully agree with you. My friend has the same 845 and it sounds quite respectful. I have auditioned it for many times. I was thinking about getting one but may leave it to the future when I finish my existing projects.

Which components have you upgraded to give better sound? How big is the improvement? I am curious because I may really get one or build one someday.

Regards,
Bill
 
Bill,
Music Angel 845SET mods here:
http://www.audioasylum.com/forums/set/messages/37030.html

Am I allowed to post a link to another forum? - well what the heck!

Since then I have replaced the 6N9P with some NOS 6SL7 with a good improvement
BUT
replacing the 6N8P with NOS 6SN7 was the real winner - huge improvement.

Cheers,
Ian
B.T.W I did a full schematic trace and took some digital photos of the guts which I have put into a word document "Service Manual".
email me if you'd like me to send it to you - much too big to post here.
ALSO
If you search for Rarefaction the one you want is about the 4th hit - the fist couple are for a site for gentlemen of a particular sexual preference and have no info on valve amps (I assume).
 
Thanks for the replies guys.

I think at this stage I'll go with the silicon chip design so I can see how they are and then I'll move up to something better at a later stage.

I have heard that some people don't like the switched mode power supply used to step the voltage up from 12V DC to 260V DC.

So does anyone have any schematics or web sites that you could refer me to to build a better supply? I like the idea of the silicon chip version in that you can use a 12v DC "wallmart" plugpack rather than having to use a transformer, with rectifier etc (and have heard that this is a better way so I won't get an hum problems).
 
For the power supply modification use 2 identical transformers wired back to back.

That is:
Use a 240V to 12V transformer plus rectifier and filter to generate the 12V DC for the valve heaters. Then connect the 12V secondary of a second transformer to the secondary of this transformer (before the heater supply rectifier) wich will give you about 240V AC on its primary which (you use as a secondary). You can rectify and filter for the high voltage supply. Simple and you get 2 levels of isolation from the mains.

Cheers,
Ian
 
Ian,

Thanks for the reply.

I've never seen that trick before (using the transformer the other way around), and it sounds easy enough. Although my question now is, when the 240v is rectified, won't the output be around 339V (excluding diode losses etc)? Hows does this affect the circuit (which requires 260V)?
 
Theoretical maximum with no load is as you say 339V.

With losses thru 2 transformers and with a load it will probably be closer to 300V which is close enough to 260V for the circuit to work without modification. You can add a dropping resistor and an extra filter capacitor into the High Voltage line to reduce that if you are at all concerned. Or alternatively can can put dropping resistors into each of the AC High Voltage wires before the rectifier which will achieve the same thing.

BTW don't forget basic safety - extra fuse in the feed to the second transformer is a GOOD idea. You were already going to fuse the mains side of the first transforemer - right?

Cheers,
Ian
 

David170

Member
2008-02-16 12:23 pm
Hi there,
I built one of those preamps a while back, I tried recording through my Mbox and it worked fine for guitar, though I think its a bit too accurate and clean perhaps for getting good gat tone. But I reckon it could be ok as a mic preamp. My question is, if I built this phantom power supply [ http://www.tangible-technology.com/power/Phantom_frying.html ] , could I hook em up directly and record with a condenser? I've scoured the forums and apparently might need some kind of transformer. Maybe more.. If you could help me out on this I'd be super appreciative, coz, really, I have no idea. :cool:

Thanks

Dave
 
gingertube said:
Theoretical maximum with no load is as you say 339V.

With losses thru 2 transformers and with a load it will probably be closer to 300V which is close enough to 260V for the circuit to work without modification. You can add a dropping resistor and an extra filter capacitor into the High Voltage line to reduce that if you are at all concerned. Or alternatively can can put dropping resistors into each of the AC High Voltage wires before the rectifier which will achieve the same thing.

BTW don't forget basic safety - extra fuse in the feed to the second transformer is a GOOD idea. You were already going to fuse the mains side of the first transforemer - right?

Cheers,
Ian


Ian's suggestion can be extrapolated to use CLC filtration, with a smallish (say 1.5 muF.) cap. in the 1st position. The 30 H./40 mA. Hammond 157G will do very nicely for this project and have further use, in the future.

Use UF4007 diodes in the rectifier bridge. While only slightly more expensive than 1N4007s, UF4007s are MUCH less noisy.
 
I built that 12AX7 preamp and even ordered the PCBs from RCS Radio where you guys live (Australia). It's my first tube project, and overcoming the challenges presented by use of tubes in a vehicle was fairly easy using this design, with some additional circuits for vehicle-specific issues (ground loop isolation, heating timer, etc.).

I enjoyed the preamp, as long as I wasn't using an amplifer with too many preamp stages or "processing". That took away the valve sound almost entirely.

But I had problems with high-pitched noise with the transformer. Otherwise the SMPS works pretty well!

"The problem with this kit is that the switch mode PSU is noisy and you have to be nearly one metre away not to hear it when your room is quiet."

I knew it!! I was sure I wasn't the only one with this problem!! I emailed the editor @ Silicon Chip and he assured me that "that wasn't a problem with this type of design".

Uh-huh, sure. :dodgy: I have the exact same core, bobbin, and winding pattern...?

I will say this though: the quality of the PCBs from RCS was not fantastic [copper traces appear to be 'wrinkled'] and I had to modify slightly (drilled holes & used 9-pin ceramic sockets from ebay, added thru-hole RCA jacks, etc.).

Overall in my opinion a good starting point. Got them working right away.. :) ..just my $.02
 
To everyone noting the noise SMPS for the Silicon Chip 12AX7 preamplifier:

I have been struggling with the noise issue for some time, and discovered after reading the TL494 data sheet (along with checking the waveforms) that the "DTC" pin (pin #4) appears to be missing a reference voltage, hence keeping the PWM function adjustment from working.

Also caused problems on startup when connected to another power supply for testing.

My transformer was horribly noisy until I reduced the duty cycle (it was about 90% until that point).

I simply added a 50K pot across the 10uF [between pins #2 & #4) and now it's quiet! I had to tinker with the adjustments, and I think my duty cycle is around 65%-70% now.

SILENCE IS GOLDEN!

I knew something wasn't right..... :snoopy: