Hi all,
I was thinking lately about the advantages of balanced signals. I was looking for a pohono preamp a while ago, and balanced versions (balanced inputs) came up too. This made me wondering, the signal from a cartridge is, in effect, a (floating) balanced signal. Why don't we use them?
As I don't have the budget for a balanced phono pre, I want to try an experiment with a balanced to unbalanced input convertor. in theory, I would gain 6db SNR, and a stronger signal too. My cartridge is very very low output, so the 6dB would be a good start too 🙂
the plan is to connect the tt output to the "unbalancer", and then use a very very short (unbalanced) cable to the phono pre.
I made up a circuit, based on a TL071 op amp per channel,
I'd like some feedback on this design, can I improve this, would it work to begin with? I want to do this as an experiment, and improve later on if I find that it offers me advantages, and maybe build myself a balanced phono preamp alltogether.
I still need to design a power supply, will be using a transformer with center tap. I also think I made a mistake with the mass connections on the inputs. I connected them to the ground/center tap, but I think it would have been better if they were a chassis ground.
Anyhow, Thanks in advance,
Keep them spinning,
Regards,
Bert.
I was thinking lately about the advantages of balanced signals. I was looking for a pohono preamp a while ago, and balanced versions (balanced inputs) came up too. This made me wondering, the signal from a cartridge is, in effect, a (floating) balanced signal. Why don't we use them?
As I don't have the budget for a balanced phono pre, I want to try an experiment with a balanced to unbalanced input convertor. in theory, I would gain 6db SNR, and a stronger signal too. My cartridge is very very low output, so the 6dB would be a good start too 🙂
the plan is to connect the tt output to the "unbalancer", and then use a very very short (unbalanced) cable to the phono pre.
I made up a circuit, based on a TL071 op amp per channel,
I'd like some feedback on this design, can I improve this, would it work to begin with? I want to do this as an experiment, and improve later on if I find that it offers me advantages, and maybe build myself a balanced phono preamp alltogether.
I still need to design a power supply, will be using a transformer with center tap. I also think I made a mistake with the mass connections on the inputs. I connected them to the ground/center tap, but I think it would have been better if they were a chassis ground.
Anyhow, Thanks in advance,
Keep them spinning,
Regards,
Bert.
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You haven't mentioned what sort of cartridge you have, but my gut feeling is that regardless of cartridge the TL-071 is not nearly quiet enough. The LM4562 and related devices would be a much better choice.
If you are using an LOMC then a transformer could well be ideal and provides balanced to unbalanced conversion if wanted as part of the deal.
If you are using an LOMC then a transformer could well be ideal and provides balanced to unbalanced conversion if wanted as part of the deal.
Thanks for the reply!
Cartridge is a Denon DL304, I am considering a better op amp too.
Transformers would be ideal, but they're fairly expensive, I think, so I wanted to try this as a cheaper alternative...
Thanks,
Bert.
Cartridge is a Denon DL304, I am considering a better op amp too.
Transformers would be ideal, but they're fairly expensive, I think, so I wanted to try this as a cheaper alternative...
Thanks,
Bert.
Hi Bert,
I run an SPU and a DL-103D and IMHO there is no currently available op-amp that is quiet enough for use at these signal levels. Output of the DL-304 is just 180uV @ 5cm/sec lateral recorded velocity which places it very squarely in the very low output camp.
A pair of step up transformers or a fully discrete head amp are the way to go. If you want to go balanced without a noise penalty you need to use a transformer. A balanced active solution is going to be at least 3dB noisier than an unbalanced solution providing the same level of overall gain.
Traditionally head amps for these sorts of signal levels have comprised multitudes of well matched reasonably low noise transistors in parallel. Oddly enough the 2N440X family as well as a number of BC series devices are quiet enough for this application and cheap.
Some people have had success paralleling low noise op-amps as well. You'll need to read up on how to do this as it is not quite as simple as you might think. (At minimum a small series resistor is required on the output of each paralleled op-amp to assure that they do not fight over slight OLG and input related offset mismatches..) And the big caveat is I do not have a sense of how good this might sound.
I'd recommend roughly 22 - 26dB of gain with this cartridge, and if active should be unbalanced. I'd get transformers - something from Lundahl or look for partridge 972 on eBay.
I run an SPU and a DL-103D and IMHO there is no currently available op-amp that is quiet enough for use at these signal levels. Output of the DL-304 is just 180uV @ 5cm/sec lateral recorded velocity which places it very squarely in the very low output camp.
A pair of step up transformers or a fully discrete head amp are the way to go. If you want to go balanced without a noise penalty you need to use a transformer. A balanced active solution is going to be at least 3dB noisier than an unbalanced solution providing the same level of overall gain.
Traditionally head amps for these sorts of signal levels have comprised multitudes of well matched reasonably low noise transistors in parallel. Oddly enough the 2N440X family as well as a number of BC series devices are quiet enough for this application and cheap.
Some people have had success paralleling low noise op-amps as well. You'll need to read up on how to do this as it is not quite as simple as you might think. (At minimum a small series resistor is required on the output of each paralleled op-amp to assure that they do not fight over slight OLG and input related offset mismatches..) And the big caveat is I do not have a sense of how good this might sound.
I'd recommend roughly 22 - 26dB of gain with this cartridge, and if active should be unbalanced. I'd get transformers - something from Lundahl or look for partridge 972 on eBay.
Or, what would happen if I used a better opamp, and preamplify the signal already, to, MM level (say, 5mV). If possible, I might also lower the input impedance, to say, 5k total, so 2,5K resistors. If, however, that would work. I will need bigger input capacitors then though...
Just an idea...
Looking at transformers now too...
Thanks,
Bert.
Just an idea...
Looking at transformers now too...
Thanks,
Bert.
Are the Deane Jensen discrete op-amps still available? They are constructed of discrete components, including ICs which are massively parallel transistors internally. They also make about the best small-signal audio transformers you will ever find. But nothing cheap.
Have a look and seriously consider SALAS RIAA.
The thread is now quite long and includes all the build suggestions.
Support from SALAS is very good.
The thread is now quite long and includes all the build suggestions.
Support from SALAS is very good.
> the signal from a cartridge is, in effect, a (floating) balanced signal.
Traditionally one pin is strapped to cable or arm shield, forcing unbalanced. This can sometimes be removed.
> in theory, I would gain 6db SNR
How? It is the same signal, now split across two input devices. The natural result is 3db poorer S/N. Good design can give the same S/N.
You may be thinking of active amplifiers where "balanced" often means one unbalanced output plus an inverter to give additional output and balance. The cart total output is what it is.
That's for thermal hiss. Buzzz picked-up in the cable may be reduced 20dB or more. However hum picked-up in the coils generally is not changed.
> feedback on this design
The one op-amp unbal has large input resistors adding thermal hiss. The cartridge is 5K-50K of signal; you have added 20K of resistor hiss.
(Also the diff-amp input impedance is not "47K" nominal load.)
Counting very roughly on thumbs:
cart, 20K: 2uV hiss
20K diff-amp: 2uV hiss
TL072: 1.4uV hiss
NE5532: 0.7uV hiss
cart and plain '072 preamp: 2.5uV hiss
cart and plain 5532 preamp: 2.1uV hiss
cart and 10K+10K+'072 diff-amp: 3.2uV hiss
Changing diff-amp input resistors to a few K gets diff-amp hiss below cart hiss, but puts cart load far below nominal 47K.
You must go to 2-opamp or 3-opamp (or instrumentation amp) to get high input resistance with low noise resistance. Microphone amplifiers re-scaled for the higher impedance of a phono cart.
Mostly, there is little advantage to taking phono carts "floating" or "balanced". Hiss is not improved, and can be worse. Hum/buzz can be reduced, but cable length MUST be short (to limit capacitance loading) and the area "should" be "clean" (no arc-welders, fluorescent lamps, heavy power lines).
Traditionally one pin is strapped to cable or arm shield, forcing unbalanced. This can sometimes be removed.
> in theory, I would gain 6db SNR
How? It is the same signal, now split across two input devices. The natural result is 3db poorer S/N. Good design can give the same S/N.
You may be thinking of active amplifiers where "balanced" often means one unbalanced output plus an inverter to give additional output and balance. The cart total output is what it is.
That's for thermal hiss. Buzzz picked-up in the cable may be reduced 20dB or more. However hum picked-up in the coils generally is not changed.
> feedback on this design
The one op-amp unbal has large input resistors adding thermal hiss. The cartridge is 5K-50K of signal; you have added 20K of resistor hiss.
(Also the diff-amp input impedance is not "47K" nominal load.)
Counting very roughly on thumbs:
cart, 20K: 2uV hiss
20K diff-amp: 2uV hiss
TL072: 1.4uV hiss
NE5532: 0.7uV hiss
cart and plain '072 preamp: 2.5uV hiss
cart and plain 5532 preamp: 2.1uV hiss
cart and 10K+10K+'072 diff-amp: 3.2uV hiss
Changing diff-amp input resistors to a few K gets diff-amp hiss below cart hiss, but puts cart load far below nominal 47K.
You must go to 2-opamp or 3-opamp (or instrumentation amp) to get high input resistance with low noise resistance. Microphone amplifiers re-scaled for the higher impedance of a phono cart.
Mostly, there is little advantage to taking phono carts "floating" or "balanced". Hiss is not improved, and can be worse. Hum/buzz can be reduced, but cable length MUST be short (to limit capacitance loading) and the area "should" be "clean" (no arc-welders, fluorescent lamps, heavy power lines).
> Cartridge is a Denon DL304
Apologies for not seeing this detail sooner.
Denon moving-coil cartridge
* Denon DL-304 Specifications
* Output: 0.18mV
* Output impedance: 40 ohms
Compare with a "normal" cart:
Stanton 681 moving iron
* 681 EEE Mk II Technical Specifications:
* Output 2.5mV
* DC Resistance: 1300 ohms
* Inductance: 930 mH
* Load Resistance: 47k Load
* Capacitance: 275pF
1/14th of the output, ~~1/200th of the impedance (the Stanton's inductance rises in the treble; where hiss lives).
As Kevinkr says, 40 ohms is VERY low noise resistance for available active devices. Because of the low source impedance, an obvious answer is a step-up transformer. This moves some interface issues from the cartridge to the transformer, where a more elegant solution may be possible; it does add considerable cost.
While moving coil impedances are "too low" for best noise direct to active devices, cart winding hiss is not the dominant hiss when the needle is on the vinyl. We are often happy with an "imperfect" electrical noise level. There are dozens of published pre-pre-amps giving flat gain with useful noise.
Even Nat Semi published one, and very neat. It was scaled for more gain than you want; I've noted a change for gain reasonable for your use.
http://www.national.com/an/AN/AN-222.pdf
As KevinKR says, the 2N4401 often works well in such schemes.
W. Marshall Leach, Jr. published several ideas:
Moving Coil Cartridge Head Amps
Apologies for not seeing this detail sooner.
Denon moving-coil cartridge
* Denon DL-304 Specifications
* Output: 0.18mV
* Output impedance: 40 ohms
Compare with a "normal" cart:
Stanton 681 moving iron
* 681 EEE Mk II Technical Specifications:
* Output 2.5mV
* DC Resistance: 1300 ohms
* Inductance: 930 mH
* Load Resistance: 47k Load
* Capacitance: 275pF
1/14th of the output, ~~1/200th of the impedance (the Stanton's inductance rises in the treble; where hiss lives).
As Kevinkr says, 40 ohms is VERY low noise resistance for available active devices. Because of the low source impedance, an obvious answer is a step-up transformer. This moves some interface issues from the cartridge to the transformer, where a more elegant solution may be possible; it does add considerable cost.
While moving coil impedances are "too low" for best noise direct to active devices, cart winding hiss is not the dominant hiss when the needle is on the vinyl. We are often happy with an "imperfect" electrical noise level. There are dozens of published pre-pre-amps giving flat gain with useful noise.
Even Nat Semi published one, and very neat. It was scaled for more gain than you want; I've noted a change for gain reasonable for your use.
http://www.national.com/an/AN/AN-222.pdf
As KevinKR says, the 2N4401 often works well in such schemes.
W. Marshall Leach, Jr. published several ideas:
Moving Coil Cartridge Head Amps
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OT: glad to see you back PRR, now please look at the tubes forum where there is a big excitement goin on re: concertina phase splitters.....thanks....😀
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