Re: Your questions
1. Which type of tube should I design for, assuming I want a general purpose, warm dual-channel preamp?
A tube that is ubiquitous and is readily avaliable and not too expensive. Also the higher the transconductance of the first gain stage the lower the noise. There are many tubes on the shelves of places like ANTIQUE ELECTRONIC SUPPLY that are just sitting there and have only been used in TV sets that are GREAT audio performers.
2. How do I convert balanced audio into unbalanced for the tube and then back to balanced?
The best way to convert to balanced and unbalanced is a good audio transformers. It takes three op amps to do the same thing. And op-amps are op-amps!
3. Should I even bother doing my own design or should I steal someone else's?
If you're a member of mensa or an A+ student, and can beat anyone at chess then go for it. If you're not then as a first project I would take it a little slower and learn as you go. We are all disadvantaged by what we don't know. I would choose a topology that is STANDARD and has stood the test of time like the PULTEK MB1. There is also a fine PULTEK EPQ1A equalizer on the web. Copy the PULTEK's chassis layout or observe the fine points. You want a stereo preamp, just build two! Keep the power transformer as far away from the input stage as possible. Shield all the high gain tubes like a 12ax7a/7025. Use DC heaters on all high gain tubes. Power amp or output stages can be lit with AC filaments but tightly twist or shield AC filament lines. It will take you some time to mail order and assemble all the proper parts and assemble it. Much of the success of the project depends on how neat you are in chassis layout, drilling, reaming, punching, sawing, filing, wire routing and how well you can solder. Learn to solder WELL! If you do this in under 6 months I would say you are going too fast!
4. What other subjects should I read up on besides tubes and then general electronics?
Become familiar with the essentials of CLASS A operation. IN MY HUMBLE OPINION there has yet to be any improvement on CLASS A operation. It's inefficient but it sounds better than any other class of operation. PHYSICS and ART. Try to balance both sides of your brain. Electrical Engineers are too math oriented. Artists are too subjective. One can't see the forest for the trees and the other can't see the trees for the forest. Good luck! Ray Hughes
1. Which type of tube should I design for, assuming I want a general purpose, warm dual-channel preamp?
A tube that is ubiquitous and is readily avaliable and not too expensive. Also the higher the transconductance of the first gain stage the lower the noise. There are many tubes on the shelves of places like ANTIQUE ELECTRONIC SUPPLY that are just sitting there and have only been used in TV sets that are GREAT audio performers.
2. How do I convert balanced audio into unbalanced for the tube and then back to balanced?
The best way to convert to balanced and unbalanced is a good audio transformers. It takes three op amps to do the same thing. And op-amps are op-amps!
3. Should I even bother doing my own design or should I steal someone else's?
If you're a member of mensa or an A+ student, and can beat anyone at chess then go for it. If you're not then as a first project I would take it a little slower and learn as you go. We are all disadvantaged by what we don't know. I would choose a topology that is STANDARD and has stood the test of time like the PULTEK MB1. There is also a fine PULTEK EPQ1A equalizer on the web. Copy the PULTEK's chassis layout or observe the fine points. You want a stereo preamp, just build two! Keep the power transformer as far away from the input stage as possible. Shield all the high gain tubes like a 12ax7a/7025. Use DC heaters on all high gain tubes. Power amp or output stages can be lit with AC filaments but tightly twist or shield AC filament lines. It will take you some time to mail order and assemble all the proper parts and assemble it. Much of the success of the project depends on how neat you are in chassis layout, drilling, reaming, punching, sawing, filing, wire routing and how well you can solder. Learn to solder WELL! If you do this in under 6 months I would say you are going too fast!
4. What other subjects should I read up on besides tubes and then general electronics?
Become familiar with the essentials of CLASS A operation. IN MY HUMBLE OPINION there has yet to be any improvement on CLASS A operation. It's inefficient but it sounds better than any other class of operation. PHYSICS and ART. Try to balance both sides of your brain. Electrical Engineers are too math oriented. Artists are too subjective. One can't see the forest for the trees and the other can't see the trees for the forest. Good luck! Ray Hughes
GR:
I cannot simply state what gain I need. Mics break, new ones are purchased, and I didn't plan on using this preamp for only one mic.
I've been engineering for about a year now.
I saw that pultec earlier and I REALLY like it, I Thought I read there wasn't a schematic but now I am seeing that.
That synthetic reverb ( http://www.pat2pdf.org/patents/pat2872515.pdf ) looks cool, maybe thats worth a try as well.
I cannot simply state what gain I need. Mics break, new ones are purchased, and I didn't plan on using this preamp for only one mic.
I've been engineering for about a year now.
I saw that pultec earlier and I REALLY like it, I Thought I read there wasn't a schematic but now I am seeing that.
That synthetic reverb ( http://www.pat2pdf.org/patents/pat2872515.pdf ) looks cool, maybe thats worth a try as well.
Re: Gain
Shoot for a gain of 100 or 40db in the preamp itself (tube circuit) with another 20 db of gain in the input transformer which gives you 60db of gain. But build in a gain control, either a pot or a shorting selector switch like 10 positions with fixed resistors. Pots can become noisy and scratchy. Ray
Shoot for a gain of 100 or 40db in the preamp itself (tube circuit) with another 20 db of gain in the input transformer which gives you 60db of gain. But build in a gain control, either a pot or a shorting selector switch like 10 positions with fixed resistors. Pots can become noisy and scratchy. Ray
Re: Microphones
I'm not asking for an argument but I really don't see what people see in a 57 (beta or standard). I guess it's different strokes for different folks. It's got this god awful peak around 10k that makes the human voice sound like they are talking through a trashcan. I guess that's why "giitar pickrs" like it. The only good thing I can say for it is that it is robust, doesn't need a battery and you could use it as a weapon in a riot. (Maybe the secret service has such plans under the table just in case. Those 57s on the podium can be pressed into media boppers.) Or maybe let's let G.W. make a shrine for it in his presidential museum and he can pray to it every day and MAKE BELIEVE he's telling the truth. Once you use a real condenser, you will NEVER go back to any dynamic microphone. Yes, they are more expensive, but you will never regret the purchase. If fact it's the most important purchase you can make because the character of the sound it formed at the first mic-amplifier stage. Ray Hughes
I'm not asking for an argument but I really don't see what people see in a 57 (beta or standard). I guess it's different strokes for different folks. It's got this god awful peak around 10k that makes the human voice sound like they are talking through a trashcan. I guess that's why "giitar pickrs" like it. The only good thing I can say for it is that it is robust, doesn't need a battery and you could use it as a weapon in a riot. (Maybe the secret service has such plans under the table just in case. Those 57s on the podium can be pressed into media boppers.) Or maybe let's let G.W. make a shrine for it in his presidential museum and he can pray to it every day and MAKE BELIEVE he's telling the truth. Once you use a real condenser, you will NEVER go back to any dynamic microphone. Yes, they are more expensive, but you will never regret the purchase. If fact it's the most important purchase you can make because the character of the sound it formed at the first mic-amplifier stage. Ray Hughes
No arguments here. I'm not a big fan of it either, but it is a ubiquitous mic and most recording guys will be familiar with its sound. But 57's have their uses. I have a bunch, but I only use it for some very specific things (mainly on drum kits, etc -- even then it gets used less and less as my mic locker gets bigger). Give me and SM7b or EV RE20 or a Heil PR-20 over an SM57 (or even most condensers) any day.
The most apt (but trite) analogy is -- "mics to recording engineers are like brushes to a painter", etc.
I would definitely use a condenser like a U87 or Microtech-Geffel M-930 on a soft, smokey female jazz singer, but not on more strident male rock vocals, for example. For acoustic guitar or classical violin: I would use Earthworks or DPA mics in XY or ORTF. Both these mics have electret(!) capsules.
And not to argue about condensers vs dynamics - "just my opinion" - at around the $400 range: I would spring for an SM7b over any condenser. You just have to make sure you have a nice preamp. (A winning combination is an SM7b into a Great River 1NV (or any other Neve preamp clone)).
Cheers!
The most apt (but trite) analogy is -- "mics to recording engineers are like brushes to a painter", etc.
I would definitely use a condenser like a U87 or Microtech-Geffel M-930 on a soft, smokey female jazz singer, but not on more strident male rock vocals, for example. For acoustic guitar or classical violin: I would use Earthworks or DPA mics in XY or ORTF. Both these mics have electret(!) capsules.
And not to argue about condensers vs dynamics - "just my opinion" - at around the $400 range: I would spring for an SM7b over any condenser. You just have to make sure you have a nice preamp. (A winning combination is an SM7b into a Great River 1NV (or any other Neve preamp clone)).
Cheers!
I really like the SM57s and SM58s for exactly what they are... rugged, decent-sounding microphones that are American made, very predictable, easily replaceable, capable of handling high SPLs, and in a pinch, you can drive nails with the things. 
Condenser mics they ain't... they're great for live sound and they certainly have their place in the studio but you really need the right kind of preamplifier to make them truly come alive in that sort of setting. Someone with good ears could record an entire album using nothing but 57s and 58s if they know what they're doing... I know, I've heard it done before.
I agree that 60dB is a reasonable goal to shoot for and should be fairly easy to obtain using a class A diffamp with no negative feedback, and an input transformer to pick up a bit of voltage gain up front. I actually just got done building something very similar to this, but with no input transformer.... I used a Lundahl LL1689 P-P transformer in the output and made the secondary windings switchable, a bit more versatile as it gives you both 6dB clicks and switchable output impedance.
Condenser mics they ain't... they're great for live sound and they certainly have their place in the studio but you really need the right kind of preamplifier to make them truly come alive in that sort of setting. Someone with good ears could record an entire album using nothing but 57s and 58s if they know what they're doing... I know, I've heard it done before.
I agree that 60dB is a reasonable goal to shoot for and should be fairly easy to obtain using a class A diffamp with no negative feedback, and an input transformer to pick up a bit of voltage gain up front. I actually just got done building something very similar to this, but with no input transformer.... I used a Lundahl LL1689 P-P transformer in the output and made the secondary windings switchable, a bit more versatile as it gives you both 6dB clicks and switchable output impedance.
Sadly I don't have the amazing budget of most professional studios. Therefore, I am stuck using 57's on cabs. I also throw a small body condenser on just for a different sound to mix with, but ideally I'd like to use a Royer R-122 for cabs.
But yeah, I'm going to order the parts tonight for that Pultec.
Can't wait
But yeah, I'm going to order the parts tonight for that Pultec.
Can't wait
I'm mad at Shure....
they stopped making the SM5 and the SM33 which were two of my favorite mics. The SM5 was designed as a high output mo-pic and TV dialog boom mic and was huge (looked like a RCA 77dx on steroids but with a flat black grill). Really hot output! You could stand the mic back 6 ft and still get good narration and character audio. The SM33 was (Johnnie Carson's desk mic) a ribbon but it's hi-freq was a little attenuated but very WARM. As dynamics go I like the EVRE20 and that's about it. The old EV RE55 was a surprisingly good omni. The Royer's and the Coles and my ribbon pics unless someone shows up with a RCA 44A. or a BK11A for string bass and sax. BK-5Bs are also very nice on brass. Ribbons can also be good for instruments like harpsichord that have a plucked string tizziness that close miced condensers impart to it. Ray Hughes
they stopped making the SM5 and the SM33 which were two of my favorite mics. The SM5 was designed as a high output mo-pic and TV dialog boom mic and was huge (looked like a RCA 77dx on steroids but with a flat black grill). Really hot output! You could stand the mic back 6 ft and still get good narration and character audio. The SM33 was (Johnnie Carson's desk mic) a ribbon but it's hi-freq was a little attenuated but very WARM. As dynamics go I like the EVRE20 and that's about it. The old EV RE55 was a surprisingly good omni. The Royer's and the Coles and my ribbon pics unless someone shows up with a RCA 44A. or a BK11A for string bass and sax. BK-5Bs are also very nice on brass. Ribbons can also be good for instruments like harpsichord that have a plucked string tizziness that close miced condensers impart to it. Ray Hughes
Hi Ray,
Have you tried out the SM7b? Similar in overall tone to the RE20. The SM7b is a real steal at $350USD. It's been used on vocals for Michael Jackson, Patti Labelle, Natalie Cole, John Mayer, and a lot of rock and hip-hop acts, etc. Like the RE20, the SM7b a very flattering mic.
The reason I mentioned them is that I often see folks recommend a cheap Chinese condenser in the $400 range to newbies. The SM7b or the RE20 will kick them to the curb in versatility and tone almost every time. It is a great first "real" mic.
Give the SM7b a shot!
Ryan: If you use a solid state rectifier.....
If you choose to use a solid state rectifier instead of the 6X4, then it's best to add a Reverse Recovery Spike filter between the power transformer secondary and the solid state rectifier. Silicon rectifiers produce a spike hash that gets into the noise floor. A tube rectifier would be even better. Also the 6X5 is an octal rectifier and is a better choice than the 6x4. Here is the schematic of the spike filter(scroll down):
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=71675&highlight=reverse+recovery+spike&session=
If you choose to use a solid state rectifier instead of the 6X4, then it's best to add a Reverse Recovery Spike filter between the power transformer secondary and the solid state rectifier. Silicon rectifiers produce a spike hash that gets into the noise floor. A tube rectifier would be even better. Also the 6X5 is an octal rectifier and is a better choice than the 6x4. Here is the schematic of the spike filter(scroll down):
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=71675&highlight=reverse+recovery+spike&session=
Yes, I've tried to SM7b....
and yes very similar to the RE20 which I have several of. Use them in bass drums. I wouldn't dare use a ribbon in a bass drum! Unless I'm ready to have to ribbon mic repaired! Just too much excursion that can stretch the ribbon. Also NEVER EVER check the continuity of a ribbon mic with a VOM meter. It will sever the ribbon, cut it like a knife! Also I'm skeptical of using a ribbon mic on a mixer equipped with phantom power on all mic inputs. If there is any DC-leakage on either leg of a balanced mic line and any dc off set sitting on either leg then you'll be looking to repair the ribbon mic. Use super precision (6.8K)resistors matched on those mic inputs for phantom power. Don't use carbon comp resistors as they will change value when you solder them!. Always use a ribbon mic with non phantom powered inputs as a safety issue.
and yes very similar to the RE20 which I have several of. Use them in bass drums. I wouldn't dare use a ribbon in a bass drum! Unless I'm ready to have to ribbon mic repaired! Just too much excursion that can stretch the ribbon. Also NEVER EVER check the continuity of a ribbon mic with a VOM meter. It will sever the ribbon, cut it like a knife! Also I'm skeptical of using a ribbon mic on a mixer equipped with phantom power on all mic inputs. If there is any DC-leakage on either leg of a balanced mic line and any dc off set sitting on either leg then you'll be looking to repair the ribbon mic. Use super precision (6.8K)resistors matched on those mic inputs for phantom power. Don't use carbon comp resistors as they will change value when you solder them!. Always use a ribbon mic with non phantom powered inputs as a safety issue.
The SM7b's foam grill ROTS rapidly
and yes I'm aware that it is a permanent magnet-moving coil dynamic; the slowest responding of all types of microphones because of the moving mass of the diaphragm which has to be damped. Shure's quality control could be better on this mic. Bruce Sweiden had to pick through many to find one that sounded good. It's a good radio station announce mic and if you're looking for a particular sound quality, a recording mic. I record classical, chamber music, choral, folk and seldom ever rock. That where I'm coming from. I'm sure others into rock will like it.
and yes I'm aware that it is a permanent magnet-moving coil dynamic; the slowest responding of all types of microphones because of the moving mass of the diaphragm which has to be damped. Shure's quality control could be better on this mic. Bruce Sweiden had to pick through many to find one that sounded good. It's a good radio station announce mic and if you're looking for a particular sound quality, a recording mic. I record classical, chamber music, choral, folk and seldom ever rock. That where I'm coming from. I'm sure others into rock will like it.
http://www.digilux.net/audio/uploads/PhantomPowerSchematic.pdf
Source for updating:
http://www.digilux.net/audio/uploads/PhantomPowerSchematic.vsd
Stolen from http://members.nuvox.net/~zt.robgrow/circuits/powersupply/phantompower.html
and then cleaned up.
Source for updating:
http://www.digilux.net/audio/uploads/PhantomPowerSchematic.vsd
Stolen from http://members.nuvox.net/~zt.robgrow/circuits/powersupply/phantompower.html
and then cleaned up.
An SM7 sounds like an RE-20? Bite your tongue man! The RE-20 is to radio stations what the SM-57 is to stage mics, a common affliction. I won't use them on an FM (all U87s here.) Somewhere at home I have a Stereophile CD with Holt reading one of his early articles into a couple dozen mics switched in one at a time. The RE-20 sounds broken. It's not just a mic, it's a processor too! And it's probably obvious I can't stand the things.
That's fine for the 48v phantom power.....
OK FIne for phantom. What about filaments??? You may want to investigate the Linear Technology LT1086 (adjustable) for the DC filaments to the 12ax7A. The LM317T is an old regulator and has considerable dropout from entry to exit. You've got to go into it much higher than you come out of it. Probably about 56-58volts in to get 48 volts out??? So your power transformers for phantom power has to supply 56-58 volts to the LM317. It's good but old and technology is moving fast. The LT1086 is newer but won't , I believe, go high enough for your 48v phantom power. You have to heat sink these regulators! And don't try to lite your 12au7 with DC from the same regulator at 6.3 volts or you might stress the LM317 if it's a TO220 device and you're liteing four tubes. If it's a TO3 device then you'll probably be OK heat stress wise. Just lite the 12ax7As with DC and use AC on the 12au7 filaments. For the 6.8K, 1.4w, 1% resistors at the input, get about 100, 6.8K metal film resistors from DIGI-KEY and use a 3 & 1.2 digit or better VOM and sort through the 100 resistors finding two that are matched to two decimal points. As a beginner you are gona have to follow the original schematic closely and then change and improve things as you go. If you have to buy a number of transformers there is generally deals to be had on ebay. Search for SIGNAL transformers, TRIAD transformers, UTC transformers, MAGNETEK transformers.
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1358,D1722
OK FIne for phantom. What about filaments??? You may want to investigate the Linear Technology LT1086 (adjustable) for the DC filaments to the 12ax7A. The LM317T is an old regulator and has considerable dropout from entry to exit. You've got to go into it much higher than you come out of it. Probably about 56-58volts in to get 48 volts out??? So your power transformers for phantom power has to supply 56-58 volts to the LM317. It's good but old and technology is moving fast. The LT1086 is newer but won't , I believe, go high enough for your 48v phantom power. You have to heat sink these regulators! And don't try to lite your 12au7 with DC from the same regulator at 6.3 volts or you might stress the LM317 if it's a TO220 device and you're liteing four tubes. If it's a TO3 device then you'll probably be OK heat stress wise. Just lite the 12ax7As with DC and use AC on the 12au7 filaments. For the 6.8K, 1.4w, 1% resistors at the input, get about 100, 6.8K metal film resistors from DIGI-KEY and use a 3 & 1.2 digit or better VOM and sort through the 100 resistors finding two that are matched to two decimal points. As a beginner you are gona have to follow the original schematic closely and then change and improve things as you go. If you have to buy a number of transformers there is generally deals to be had on ebay. Search for SIGNAL transformers, TRIAD transformers, UTC transformers, MAGNETEK transformers.
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1358,D1722
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