Need Help

Serendipity

I'm trying to build a tube preamp - the following is a preliminary model of what I am attempting to build. Any suggestions would be greatly appreciated, such as for the power supply, should I use a Toroidal or Iron Core Xfrmr? How well would relays work vs. a mechanical switch for the input sources? See the diagram for more details...

Pic attached

Serendipity

Thanks, any tips/hints for the design?

mrbigbluelight

Well, if you're going to catch tube-itis, you might as well go full tilt:


http://www.polkaudio.com/forums/showthread.php?t=102720

[Deleted User]

Hello,
To be honest with you I'm having a little trouble understanding the schematic. Maybe if you separated the power supply from the signal path it would help? I can't see where the signal gain happens? What is the overall circuit topology? Since you haven't specified a particular tube how can you determine the operating points? Or, the B+ requirements?
Most designs, I've seen, start off by making a list of intended requirements. What is the needed input sensitivity? What is the needed gain? What is the needed output voltage and impedance. This helps them determine what the tube specifications should be. Then they can begin determining the operating parameters, knowing the desired gain, etc.
Then the power supply can be designed after the overall circuit parameters and tube choices are made. I can highly recommend "Valve Amplifiers" by Morgan Jones third edition.
Regards, Ken

Serendipity

Kenneth Swauger wrote: »

Hello,
To be honest with you I'm having a little trouble understanding the schematic. Maybe if you separated the power supply from the signal path it would help? I can't see where the signal gain happens? What is the overall circuit topology? Since you haven't specified a particular tube how can you determine the operating points? Or, the B+ requirements?

I'm an EE major but haven't learned anything about tube audio design in school yet. So this schematic is what I "think" will work, it may not, I'm basically guessing here.

Basically, I'm undecided about the power supply design, but as you can see the signal path is up top and the output is to the upper right. I'm going for as simple a circuit as possible, this will most definitely need revision but it's a start. (I started writing this on a napkin and basically rewrote this on paper).

What tubes would you recommend? I'm not going for any gain - actually, the 150mV output of my CDP is enough to drive an amp to full gain, and a high output impedance would be ideal. Something like an output to drive an Adcom GFA-535 or pro amp would be fine here. So 200mV output or something around that area would work appropriately for this design.

Any suggestions would be appreciated, thanks!

FTGV

Serendipity wrote: »

actually, the 150mV output of my CDP is enough to drive an amp to full gain,

2 volts is the general standard for output levels of CDP's and most amps require from 750mv-1.5 v to drive them to full output.But yes even if you build yourself a tube stage with just unity gain ( basically a tube buffer) you should still have sufficient levels to drive the amp to full output.The volume control would serve to only attenuate the signal.

Serendipity

Kenneth Swauger wrote: »

Most designs, I've seen, start off by making a list of intended requirements. What is the needed input sensitivity? What is the needed gain? What is the needed output voltage and impedance. This helps them determine what the tube specifications should be. Then they can begin determining the operating parameters, knowing the desired gain, etc.
Then the power supply can be designed after the overall circuit parameters and tube choices are made. I can highly recommend "Valve Amplifiers" by Morgan Jones third edition.
Regards, Ken

What would a typical tube preamp use as far as the needed input sensitivity? I would assume something like 500mV, since that is what I've seen CD Players output, but then again I am guessing because I have limited experience with players and only have owned a few.

As for gain, anything that could drive a common amp would do. I have used a few DJ amps, such as ones from Crest Audio, Peavey, and even the Radio Shack MPA-250, and it seems like anything around/under 500mV would do, at least for Pro Audio applications. Not too familiar with home audio, but my experience comes from DJing at church/summer camps/etc.

madmax

As an EE I'll admit I do not know how to design with tubes. Classes included a one line generalization, something like "Tubes were used in the past, operate at high voltages and are obsolete". Gee, I bet the instructors could change their tune today! I can figure them out if simple enough, modify and repair but not design. There are several books available, just search something like "build tube amp books" and you will find way more than you need. From what I've read most designers in the past have looked at a given group of topographies, picked the general one that gives them what they want then do the math for their given tubes, transformers and gain requirements. Good luck!

Serendipity

FTGV wrote: »

2 volts is the general standard for output levels of CDP's and most amps require from 750mv-1.5 v to drive them to full output.But yes even if you build yourself a tube stage with just unity gain ( basically a tube buffer) you should still have sufficient levels to drive the amp to full output.The volume control would serve to only attenuate the signal.

Wow, with Pro Amps, they are plenty loud with just 200mV - I'm not going for ear bleeding volumes but can definitely fill a large space with the mixer nowhere near the 0db level. At least when I DJ'd that was what I observed.

Would a tube stage with just unity gain work fine? Like I said I might pair this up with an Adcom GFA-535, for a small 2 channel system. Or use this in the HT rig w/ projector for the front 2 channels only.

What kind of volume control would you recommend? A mechanical pot, or resistor-based control like a passive preamp?

FTGV

Serendipity wrote: »

Would a tube stage with just unity gain work fine? Like I said I might pair this up with an Adcom GFA-535, for a small 2 channel system.

I would expect that it would unless your CDP has well below standard output levels and your using ineficient speakers.

What kind of volume control would you recommend? A mechanical pot, or resistor-based control like a passive preamp?

I prefer and use a 23 position stepped attenuator in the pre of my main system.While they offer a purer signal path and perfect tracking between channels,the down side with stepped attn's is the inability to make minute volume adjustments.I have other pre's that use either Nobles or Alp's pot's the Noble having tighter tracking between channels..

Face

Alex, where did you get that voltage figure from, it sounds way low. As Fred mentioned, most are around 2v, with some as high as 5v rms and up to 10v peak.

I too prefer stepped attenuators for transparency and precise tracking.

Serendipity

Face wrote: »

Alex, where did you get that voltage figure from, it sounds way low. As Fred mentioned, most are around 2v, with some as high as 5v rms and up to 10v peak.

I too prefer stepped attenuators for transparency and precise tracking.

Well, the instruction manual for my MPA-250 amp from Radio Shack (pro amp) says the CD input sensitivity is 500mV, here:

http://www.radioshack.com/search/manualResults.jsp?sr=1&kw=mpa-250&origkw=mpa-250&kwCatId=&hideBackToAll=true

So I would think an output of 2V would overdrive the pro amp - this is NOT the case. I can turn the amp all the way up and not reach clipping, with or without gain from my DJ mixer.

[Deleted User]

Hello,
You might want to consider designing a cathode follower circuit. This is a common buffer stage design and combines high input and low output resistance. The design has a voltage gain that is slightly less than 1, less than 1k Ohm output with a nice high input resistance. Also, it is non-inverting. Most use a 12AX7, but other tubes will work. Look at the schematic for the Audio Research SP3 and SP6 and Marantz 7C pre-amps for classic cathode follower circuits.
I hope this is helpful information, Ken.

Serendipity

Kenneth Swauger wrote: »

Hello,
You might want to consider designing a cathode follower circuit. This is a common buffer stage design and combines high input and low output resistance. The design has a voltage gain that is slightly less than 1, less than 1k Ohm output with a nice high input resistance. Also, it is non-inverting. Most use a 12AX7, but other tubes will work. Look at the schematic for the Audio Research SP3 and SP6 and Marantz 7C pre-amps for classic cathode follower circuits.
I hope this is helpful information, Ken.

Thanks, so do you think the circuit I proposed will not work?

I am looking into the cathode follower circuit as well as reading about other designs (there's a lot to this that I never learned in school). Keep in mind one of the criteria is to keep this on a student's budget - as in a few hundred to $1k max, so I know cost is one of my limitations.

A circuit with a gain of 1 would work, with less than 1k output and high input resistance would be ideal for my project. I am looking into the 12AX7 tubes, I've browsed other NOS tubes I've seen online but I am confused as there are so many. Again thanks for the useful info, Ken.

[Deleted User]

Hello,
I never try to discourage anyone from trying something new, I'm all in favor of it. I've never built a stand alone buffer, only one that is part of a complete gain stage. Usually a two section amplifier with passive EQ between the stages and a cathode follower stage to reduce effects of cable loading. Your design looks like a variation of a follower, give it a try.
For me, I would look at either the 6DJ8 or 12AX7 as the tube. You can, probably, find a good sounding 6DJ8 for less money than an equal sounding 12AX7. Most of the literature, I find, endorses the cathode biased cathode follower as being best to deal with active loads. Try looking at the Sallen & Key active filters online for an example.
I've always used mechanical switches as input selectors and stepped attenuators that use precision resistors. The Electroswitch Company makes some excellent switches, but they are expensive. I believe a standard ferrous core AC transformer would be easier to find than a toroidal.
Good luck, Ken

Serendipity

Based on your suggestions, I started over and my 2nd Circuit is of a cathode follower with 48V CT transformer (might be overkill) using the 6DJ8 tube that you guys recommended.

What do you think of the diagram below?

Thanks,
Serendipity

Serendipity

Another revision based on the previous circuit:

[Deleted User]

Hello,
Where's the tube's B+? You're going to need 200/300VDC for that. Also where's the plate resistor?
Ken

Serendipity

Kenneth Swauger wrote: »

Hello,
Where's the tube's B+? You're going to need 200/300VDC for that. Also where's the plate resistor?
Ken

I haven't decided yet, since I have to pick a transformer. What would work well for this application?

As for the plate resistor, I think I will go with a 220k, not sure to some extent...

[Deleted User]

Hello,
I'm afraid your design is outside of my experience, so I don't know if it will work or not. As to AC transformers, there is Plitron for toroidal designs. Their email address is techinfo@plitron.com. Also there's Avel Transformers at 860-355-4711. For iron core there's Antique Electronic Supply 602-820-4643. Also, Hammond Manufacturing at www.hammondmfg.com. A friend of mine uses Fair Radio Sales fairadio@alpha.wcoil.com with good results.
Enjoy, Ken

Face

Alex, check your email.

madmax

http://www.diyaudio.com/forums/tubes-valves/21932-looking-tube-design-software.html

Found this which may help. There is also a program called Tube CAD I saw.
madmax

Serendipity

Kenneth Swauger wrote: »

Hello,
I'm afraid your design is outside of my experience, so I don't know if it will work or not. As to AC transformers, there is Plitron for toroidal designs. Their email address is techinfo@plitron.com. Also there's Avel Transformers at 860-355-4711. For iron core there's Antique Electronic Supply 602-820-4643. Also, Hammond Manufacturing at www.hammondmfg.com. A friend of mine uses Fair Radio Sales fairadio@alpha.wcoil.com with good results.
Enjoy, Ken

Thanks, I'm going to order the 48V CT transformer from Hammond Manufacturing, as a start. Seems to be fairly inexpensive and should get the job done.

Face - I'm not looking to buy/assemble a pre-built tube pre, the point of this is to make a design that (hopefully) sounds better.

[Deleted User]

Excellent! Keep us posted on your progress, photos are always appreciated by everyone.
Take care, Ken

Serendipity

Kenneth Swauger wrote: »

Excellent! Keep us posted on your progress, photos are always appreciated by everyone.
Take care, Ken

So - I ordered a bunch of parts from PE, along with some solder and prototyping boards. So far I've built the power supply and audio boards, attached are the pictures of what I've done so far.

Since I am no expert at soldering, it took a few tries before I got everything right. The last pic shows the experimenting I did with placing the voltage regulators and soldering them in place.

Note: Please let me know if you want to copy/use this design, a lot of hard work went into it...

Serendipity

More pics...

Again if anyone wants to copy this, please let me know first.

I don't want to see some Chinese knockoff of this design that I built

ShinAce

Sockets for the opamps! Excellent!

Serendipity

ShinAce wrote: »

Sockets for the opamps! Excellent!

Just in case I blow one, or two of 'em :eek:

ShinAce

Serendipity wrote: »

Just in case I blow one, or two of 'em :eek:

Or in case you want to roll opamps AND roll tubes!

I always enjoyed the fact that you're not exposing the chip to heat when you mount em like that. Peace of mind is important in things you build out of pride.

Face

Looks good so far. Now finish it up and tell us how it sounds.

Serendipity

Face wrote: »

Looks good so far. Now finish it up and tell us how it sounds.

I haven't finished building it yet, but I'm guessing it will sound too bright.