therecordingart
Well-known member
I want to build/buy something really cheap (under $50). Any ideas?
Phono preamps....
- Thread starter therecordingart
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ChrioN
Well-known member
http://www.geocities.com/rjm003.geo/rjmaudio/diy_pho5.html
Yes - that's a good one. And for similar setups, look up the NE5534 application notes.
Jakob E.
Jakob E.
Ptownkid
Well-known member
I built a bugle from hagtech and love it. the pcb will run you 40$ but it's well worth it.
http://www.hagtech.com/bugle.html
http://www.hagtech.com/bugle.html
bcarso
Well-known member
ChrioN said:
I'd like to have this guy show me with a double-blind test how well he can hear the power transformer "quality" ;D
I think I would upgrade the regulators to LM317/337 parts too. When you have this much gain being extracted from an opamp the power supply sensitivity can get significant.
> I think I would upgrade the regulators to LM317/337 parts too.
Well, _I_ would take those treacherous barely-stable overstrung amplifiers OUT of the supply rails. 1000uFd-220R-1000uFd in each rail ought to be ample filtering for any audio-worthy opamp chip.
And this may be in line with the specification: "really cheap (under $50)". What is requested is not super performance, but super value.
I would do it that way, passive supply and single NI amp. Inaccurate past 50KHz but works and is simple.
A few years back, I was NOT able to find anything decent ready-made in that price zone. ART Research stuff is usually good, but their $50 preamp had excess subsonic noise. Not so it would bother a PA system (the intended use), but it tripped-out a home receiver's DC protection. Most other low-price preamps are crap, even by 1970s standards.
> bugle from hagtech
Not the way I would do it, I have not heard it, but a very excellent plan and execution. Certainly 3X to 30X better than anything you buy "under $50". Probably more fault-free than most stuff in the $999 range.
Well, _I_ would take those treacherous barely-stable overstrung amplifiers OUT of the supply rails. 1000uFd-220R-1000uFd in each rail ought to be ample filtering for any audio-worthy opamp chip.
And this may be in line with the specification: "really cheap (under $50)". What is requested is not super performance, but super value.
I would do it that way, passive supply and single NI amp. Inaccurate past 50KHz but works and is simple.
A few years back, I was NOT able to find anything decent ready-made in that price zone. ART Research stuff is usually good, but their $50 preamp had excess subsonic noise. Not so it would bother a PA system (the intended use), but it tripped-out a home receiver's DC protection. Most other low-price preamps are crap, even by 1970s standards.
> bugle from hagtech
Not the way I would do it, I have not heard it, but a very excellent plan and execution. Certainly 3X to 30X better than anything you buy "under $50". Probably more fault-free than most stuff in the $999 range.
there's some interesting info on RIAA preamps:
http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_1/
http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_2/
you can try to buld the "reality check" preamp from the first part:
btw, it's my first post so hello everybody
http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_1/
http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_2/
you can try to buld the "reality check" preamp from the first part:
btw, it's my first post so hello everybody
CJ
Well-known member
What kind of Cart?
This will not work well for a low output MC, but it uses tubes, so it glows in the dark.
Taken from:
http://www.geocities.com/TimesSquare/1965/phono.html
This will not work well for a low output MC, but it uses tubes, so it glows in the dark.
Taken from:
http://www.geocities.com/TimesSquare/1965/phono.html
therecordingart said:
It's hard to scratch yourself for $50. Even when I was selling too cheap kits I got $150 for a phono preamp. but to make a respectable phono preamp any one of several new technology opamps will do well a lot simpler than throwing a bunch of discrete parts at it. I would lean toward a FET input to not load down the MM cartridge. The small errors (in theory) from a single N.I. RIAA stage are mostly above the top octave so not a major concern. I came up with a work around for that error in a preamp I published in 1978 but is required a following buffer stage. If you're going to budget two opamps per channel, you might as well use one of the popular two opamp topologies that share the gain burden and provide true pole at 75usec for RIAA equalization.
I'd be tempted to buy something really cheap then swap out opamp and probably RIAA EQ parts (I used polystyrene caps). Nowadays you need to be careful that what you buy to gut is using through hole parts, maybe shop for something older used.
JR
> I would lean toward a FET input to not load down the MM cartridge.
but-but-but... FETs have "high" noise voltage. Although... microgroove surface noise will overwhelm a starved 12AX7, or any non-dirty transistor. (I remember $9.98 phono preamps built with reject transistors, and it was not uncommon to have one side 10dB hissier than the other, luck of the draw.)
And the proposed price-limit does not beg "great" performance.
Surely even a '741 or a 5532 is not a real load to a MM needle?
Loading is not a sin. Just a compromise to work with. I happen to be reading a so-so book by a good author and found this very minimal proposal.
Yes, the 2KC pole is set by loading, the needle inductance has to be in the specified range. On many carts, it IS, because the reference load is 47K across a 12AX7 grid and you want the L-C-R freq and Q to land high in the top octave and Q=~~1. Yes, the 15K resistor adds hiss, but working hiss is more surface noise than amplifier.
I doubt it will be 2dB accurate without hand-trimming. However that ART preamp had odd 2dB errors, and was still better than anything else I saw at $49.99. The old $9.98 tanks (still around at $20-$50) were +/-5dB; often tolerable above 300Hz but prone to a shave or a bump below 100Hz.
It is missing an output cap.
but-but-but... FETs have "high" noise voltage. Although... microgroove surface noise will overwhelm a starved 12AX7, or any non-dirty transistor. (I remember $9.98 phono preamps built with reject transistors, and it was not uncommon to have one side 10dB hissier than the other, luck of the draw.)
And the proposed price-limit does not beg "great" performance.
Surely even a '741 or a 5532 is not a real load to a MM needle?
Loading is not a sin. Just a compromise to work with. I happen to be reading a so-so book by a good author and found this very minimal proposal.
Yes, the 2KC pole is set by loading, the needle inductance has to be in the specified range. On many carts, it IS, because the reference load is 47K across a 12AX7 grid and you want the L-C-R freq and Q to land high in the top octave and Q=~~1. Yes, the 15K resistor adds hiss, but working hiss is more surface noise than amplifier.
I doubt it will be 2dB accurate without hand-trimming. However that ART preamp had odd 2dB errors, and was still better than anything else I saw at $49.99. The old $9.98 tanks (still around at $20-$50) were +/-5dB; often tolerable above 300Hz but prone to a shave or a bump below 100Hz.
It is missing an output cap.
martthie_08
Well-known member
I was looking at this cheap one the other day: phono preamp They also have a Quad 405 knock off which I have been thinking about buying to rebuild my busted 405 sitting in the cellar.
PRR said:
Sorry, a reflex response after a few decades of looking for ways to improve upon already competent designs.
In my experience MM cartridge loading, like resistance and capacitance (including cables), matters for top octave frequency response.
Out of a $50 budget a few dollars for a good opamp does not seem excessive (to me), and some decent caps for the EQ. When caps are doing that much work in the middle of the audio band, their imperfections matter.
There is no reason why you can't make one that is very good, for a modest budget. The high priced stuff is mainly expensive because of packaging and marketing mojo.
I won't bother to point to suggestions since there are so many that will work adequately and execution probably matters more that which topology you settle on.
JR
CJ
Well-known member
A Masterpiece in design and performance! ... This New Deluxe STEREO PREAMPLIFIER
* Separate basic and secondary controls
* Push button selected inputs—greatly simplified operation
* Self powered—DC filaments—silicon diode rectifiers
* Deluxe styling group
* Switch-selected loudness or volume control
* Cathode follower outputs for stereo tape recording
* "Derived" center-channel monophonic output
* Rumble and scratch filters for optimum record reproduction
* Channel and phase reversal switches for convenience
* Low distortion triode stage circuit design
* For use with any stereo or mono power amplifier
* Separate basic and secondary controls
* Push button selected inputs—greatly simplified operation
* Self powered—DC filaments—silicon diode rectifiers
* Deluxe styling group
* Switch-selected loudness or volume control
* Cathode follower outputs for stereo tape recording
* "Derived" center-channel monophonic output
* Rumble and scratch filters for optimum record reproduction
* Channel and phase reversal switches for convenience
* Low distortion triode stage circuit design
* For use with any stereo or mono power amplifier
> MM cartridge loading, like resistance and capacitance (including cables), matters for top octave frequency response.
Sure. In ~~1954 a cart needed significant inductance to lever-up output above vacuum-tube noise. Also the tube was usually 3 feet away, so you had significant capacitance, both cable and tube.
For a while, each needle-winder specified different R and C loading.
The upshot was some agreement on 47K. C was not specified, but was pretty sure to be over 100pFd yet rarely over 300pFd.
Figure 200pFd and 600mH. 14.5KHz. Figure Q: 600mH at 14.5KHz is 54K8, loaded in 47K is Q=0.86. The electrical L-C-R -3dB point will be near 18KHz, which is pretty good for 1950s hi-fi. Changing to 150pFd or 250pFd has little effect below 15KHz, because the 47K tends to even things out. Omit the 47K and you get a hi-Q ring at 14.5KHz. Actually what they did was match the R to the reactance of the typical C at the high end of the audio band. The needle-maker may select L over a wide range: higher because that gives more output or less magnet-mass, lower to push the resonance toward 20KC, or balance resonance against mechanical losses in the upper octave.
There are mavricks. Grado carts had very low L, the resonance was way-high, loading was nearly unimportant. Some others are quite fussy.
Assuming a damped resonance at the top of the band is not unusual in audio.
Cowles proposes a very different plan. Hang a heavy load on the inductance. Cart output droops above 2KHz (you hope and trim). "Free" RIAA pole! The expected resonance in the last octave is heavily damped, Q=0.16. Large change in C does not matter because this is hardly a resonance at all. If you assume 100pFd (3 feet) cable, this matches the 7.5K load at 200KHz! Your electrical response is "flat" (relative to RIAA) a decade past the audio band, no 12dB/oct above 15-20KHz as with 600mH 200pFd 47K loading. You DO lose any bump-trick the cart designer expected to get by trimming L against 47K and typical C.
RIAA Noise is a complicated thing, National published a paper once, and I'm not up to running the calcs with this different plan. But note that while this "has 15K excess resistance", the normal plan is ~~5K below 2KHz rising to 47K at top resonance.
> Wonder how would a mpsa18 work..)
Read the paper. "At 30Hz the gain is determined by the transistor current gain... at least 60." Or, assuming all the caps let-go at 30Hz, you have 15K into the base and 10K load. If Hfe is unity, gain is 10K/15K= 0.67, if Hfe were 100 then gain is 100*10K/15K= 67.
Ah, I see the inevitable flaw. 1KHz gain is nearly 150K/15K= 10. We usually want 1KHz gain to be 50 or 100.
MPSA18 is Hfe 500 or more. 30Hz gain seems to be over 300, maybe 1000! WAY too high for 1KHz gain of 10! RIAA is asymtotic to a 10:1 rise each half of the audio band.
Ah... Now scale the 150K to like 1Meg, midband gain near 50, and a reasonable rise to 30Hz. C1 must go down to 290pFd. The 1M5+1M5 bias chain also scales with Hfe, should total like 5M or 10Meg, and the 0.03u may change. Although, you could replace the T with a single 1.5Meg, hang 100K base to ground to get DC operating point (you love your DC) on a good zone.
If you have a Grado, the C2 has to be near 100pFd, and may fuss-at transistor capacitance. We know the Grado did not need L-R-C resonance to tweak its top octave so you are not losing that design-trick.
It might be "better" with that hi-gain transistor.
Sure. In ~~1954 a cart needed significant inductance to lever-up output above vacuum-tube noise. Also the tube was usually 3 feet away, so you had significant capacitance, both cable and tube.
For a while, each needle-winder specified different R and C loading.
The upshot was some agreement on 47K. C was not specified, but was pretty sure to be over 100pFd yet rarely over 300pFd.
Figure 200pFd and 600mH. 14.5KHz. Figure Q: 600mH at 14.5KHz is 54K8, loaded in 47K is Q=0.86. The electrical L-C-R -3dB point will be near 18KHz, which is pretty good for 1950s hi-fi. Changing to 150pFd or 250pFd has little effect below 15KHz, because the 47K tends to even things out. Omit the 47K and you get a hi-Q ring at 14.5KHz. Actually what they did was match the R to the reactance of the typical C at the high end of the audio band. The needle-maker may select L over a wide range: higher because that gives more output or less magnet-mass, lower to push the resonance toward 20KC, or balance resonance against mechanical losses in the upper octave.
There are mavricks. Grado carts had very low L, the resonance was way-high, loading was nearly unimportant. Some others are quite fussy.
Assuming a damped resonance at the top of the band is not unusual in audio.
Cowles proposes a very different plan. Hang a heavy load on the inductance. Cart output droops above 2KHz (you hope and trim). "Free" RIAA pole! The expected resonance in the last octave is heavily damped, Q=0.16. Large change in C does not matter because this is hardly a resonance at all. If you assume 100pFd (3 feet) cable, this matches the 7.5K load at 200KHz! Your electrical response is "flat" (relative to RIAA) a decade past the audio band, no 12dB/oct above 15-20KHz as with 600mH 200pFd 47K loading. You DO lose any bump-trick the cart designer expected to get by trimming L against 47K and typical C.
RIAA Noise is a complicated thing, National published a paper once, and I'm not up to running the calcs with this different plan. But note that while this "has 15K excess resistance", the normal plan is ~~5K below 2KHz rising to 47K at top resonance.
> Wonder how would a mpsa18 work..)
Read the paper. "At 30Hz the gain is determined by the transistor current gain... at least 60." Or, assuming all the caps let-go at 30Hz, you have 15K into the base and 10K load. If Hfe is unity, gain is 10K/15K= 0.67, if Hfe were 100 then gain is 100*10K/15K= 67.
Ah, I see the inevitable flaw. 1KHz gain is nearly 150K/15K= 10. We usually want 1KHz gain to be 50 or 100.
MPSA18 is Hfe 500 or more. 30Hz gain seems to be over 300, maybe 1000! WAY too high for 1KHz gain of 10! RIAA is asymtotic to a 10:1 rise each half of the audio band.
Ah... Now scale the 150K to like 1Meg, midband gain near 50, and a reasonable rise to 30Hz. C1 must go down to 290pFd. The 1M5+1M5 bias chain also scales with Hfe, should total like 5M or 10Meg, and the 0.03u may change. Although, you could replace the T with a single 1.5Meg, hang 100K base to ground to get DC operating point (you love your DC) on a good zone.
If you have a Grado, the C2 has to be near 100pFd, and may fuss-at transistor capacitance. We know the Grado did not need L-R-C resonance to tweak its top octave so you are not losing that design-trick.
It might be "better" with that hi-gain transistor.
Have fun reinventing that wheel for the Nth time.
I really like that idea of passively executing the 75 usec pole before the active electronics, if you could be confident in cart characteristics, or are willing to dial it in. My last attempt to polish that tu__ , oops fine playback medium, involved a passive 75 usec pole immediately following an open loop input gain stage. I liked the way you could hit the input with unlimited edge rate and it would harmlessly roll it off like a LPF.
I used to sell a little switched capacitor load add-on PCB so customers could dial in their cart loading with minimal pain. It might be even easier to just tweak phono cable length, not that I have much faith in the quality of that capacitance.
My judgement is that some of the variability between subjective reviews of phono preamps was more due to the reviewers nominal cartridge loading, than actual circuitry or EQ accuracy differences, but its pretty much ancient history now. There are probably fewer MM carts available today so it might be easier to design for just one or two.
JR
I really like that idea of passively executing the 75 usec pole before the active electronics, if you could be confident in cart characteristics, or are willing to dial it in. My last attempt to polish that tu__ , oops fine playback medium, involved a passive 75 usec pole immediately following an open loop input gain stage. I liked the way you could hit the input with unlimited edge rate and it would harmlessly roll it off like a LPF.
I used to sell a little switched capacitor load add-on PCB so customers could dial in their cart loading with minimal pain. It might be even easier to just tweak phono cable length, not that I have much faith in the quality of that capacitance.
My judgement is that some of the variability between subjective reviews of phono preamps was more due to the reviewers nominal cartridge loading, than actual circuitry or EQ accuracy differences, but its pretty much ancient history now. There are probably fewer MM carts available today so it might be easier to design for just one or two.
JR
CJ
Well-known member
Rumor has it that you can strap a UTC A-10 for an MC cart.
Kind of a poor man's K 251 D, or whatever that expensive Peerless MC transformer is.
Wire it for 35 ohms or some weirdness.
What about tone arm capacitance?
Check AES if you want volumes on turntable science.
Kind of a poor man's K 251 D, or whatever that expensive Peerless MC transformer is.
Wire it for 35 ohms or some weirdness.
What about tone arm capacitance?
Check AES if you want volumes on turntable science.
MC source was even lower than 35 ohm IIRC but transformers were popular.
I was able to make a respectable MC amp w/o transformer using 2SB737s bipolar transistors, back in 1980 or so.
They are obsolete now, but still available in small numbers over WWW. (I have a small stash for personal projects).
The low impedance of MC carts makes cable capacitance insignificant.
JR
I was able to make a respectable MC amp w/o transformer using 2SB737s bipolar transistors, back in 1980 or so.
They are obsolete now, but still available in small numbers over WWW. (I have a small stash for personal projects).
The low impedance of MC carts makes cable capacitance insignificant.
JR
Without studying the details, it looks fine... Good modern opamp, etc.
JR
JR
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