After reading a lot of positive reviews about this little known vintage radio shack phono pre I had to give it a try. Someone was selling it with modified/upgraded caps so I got one and I must say it was quite good. So I started looking into it see what other areas of improvement exist. One of the things that I wanted to evaluate (and where I need your help) is with the biasing and gain split between the two stages.
Question 1: The gain seems to be split in a very diverse amount between the two stages. If I am not wrong the first stage has several hundered (600?) of gain and the second stage is about 20 ? Does that seem right ? is it typical of such two stage phono pres to have the gain split like this? I felt a little more balanced gain split would help with noise and distortion performance. It should allow TR1 to be biased a bit away from the fringe limits. That brings me to my next question -
Question 2: TR1 voltages are quite low. Vc=1.82V, Vce=1.78v, Vb=0.61v, Vbc=1.15v and Vbe=0.51v.
I doubt its operating at its best. Couldnt find any meaningful datasheets for the transistor so going out on a limb here. Do you think it would help rebiasing TR1 (and thereby TR2 to keep total gain of the preamp same) to operate at somewhat higher voltages would help improve its performance in its operating range for such small signal voltages and currents ?
Question 3: What is 0.835V written in between TR1 and TR2 ? Is it Ve or Vce or what ?
Question 1: The gain seems to be split in a very diverse amount between the two stages. If I am not wrong the first stage has several hundered (600?) of gain and the second stage is about 20 ? Does that seem right ? is it typical of such two stage phono pres to have the gain split like this? I felt a little more balanced gain split would help with noise and distortion performance. It should allow TR1 to be biased a bit away from the fringe limits. That brings me to my next question -
Question 2: TR1 voltages are quite low. Vc=1.82V, Vce=1.78v, Vb=0.61v, Vbc=1.15v and Vbe=0.51v.
I doubt its operating at its best. Couldnt find any meaningful datasheets for the transistor so going out on a limb here. Do you think it would help rebiasing TR1 (and thereby TR2 to keep total gain of the preamp same) to operate at somewhat higher voltages would help improve its performance in its operating range for such small signal voltages and currents ?
Question 3: What is 0.835V written in between TR1 and TR2 ? Is it Ve or Vce or what ?
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I would guess its Ve on TR1 and missing an arrow.
Since that is the only leg missing a value.
The arrows look to have been penned in by hand.
Although, that might not make sense as Vb is 0.5
Note 0.835 is in a different font so has been added later.
As you have the device, have a measure.
Since that is the only leg missing a value.
The arrows look to have been penned in by hand.
Although, that might not make sense as Vb is 0.5
Note 0.835 is in a different font so has been added later.
As you have the device, have a measure.
I edited my original post with the voltages for TR1 and here they are as well - Vc=1.82V, Vce=1.78v, Vb=0.61v, Vbc=1.15v and Vbe=0.51v.
Here's the datasheet for 2sc1570 - https://www.datasheets360.com/pdf/-1718964263472905034
Here's the datasheet for 2sc1570 - https://www.datasheets360.com/pdf/-1718964263472905034
Q1: don't forget the emitter of TR2 is decoupled to ground by C09, so its AC gain will be high. The DC and AC gain of TR1 is ~600, so the pair form a 'single' high-gain (self-biased) amplifier. With the RIAA feedback network C11/C13/R17/R19(/R07) connected from its output to its inverting input, it forms a single-stage phono amplifier rather than two-stage.
Q2: the DC feedback around TR2 will set Vc_TR1 at ~(800/330).Vb_TR1 + Vbe_TR2, ensuring adequate Vce_TR1. AC signals at TR1 b (due to the input signal) and c (due to the high AC gain of TR2) will be small, so there should be no issues with its operating conditions.
Q2: the DC feedback around TR2 will set Vc_TR1 at ~(800/330).Vb_TR1 + Vbe_TR2, ensuring adequate Vce_TR1. AC signals at TR1 b (due to the input signal) and c (due to the high AC gain of TR2) will be small, so there should be no issues with its operating conditions.
Hallo Percy,
your circuit reminds me a bit of the first VALVO equalizers, battery powered they sounded really good.
Maybe you like our two-stage approach, it has clear advantages over the ancient two transistor version, have a look at the related thread:
OREAD MM-EQ
I'm in a bit of a hurry and very busy at work, so I won't be able to answer your questions about the above circuit until later.
your circuit reminds me a bit of the first VALVO equalizers, battery powered they sounded really good.
Maybe you like our two-stage approach, it has clear advantages over the ancient two transistor version, have a look at the related thread:
OREAD MM-EQ
I'm in a bit of a hurry and very busy at work, so I won't be able to answer your questions about the above circuit until later.
Would the shot noise be as much, if not more, than noise from those old resistors and capacitors ? (although in my unit they are all upgraded to modern ones) And how much low collector current could it go ? The entire unit in operation drains about 3.5ma.
I realize I didnt mention that one of the reasons I started thinking in the re-bias direction is that I increased the supply voltage from 28v to 36v and it made a substantial improvement in distortion (for the same input level) and headroom. The difference was also quite audible.
Before I move on to other preamps I am too tempted to drop R05 (tr1) from 220k to maybe 180k or 150k, increase supply to 34-36v and see (rather hear) what happens. Last call to me stop me from doing that 🙂
I realize I didnt mention that one of the reasons I started thinking in the re-bias direction is that I increased the supply voltage from 28v to 36v and it made a substantial improvement in distortion (for the same input level) and headroom. The difference was also quite audible.
Before I move on to other preamps I am too tempted to drop R05 (tr1) from 220k to maybe 180k or 150k, increase supply to 34-36v and see (rather hear) what happens. Last call to me stop me from doing that 🙂
One thing that might make an audible improvement is to jumper C15. It is a rumble filter with a -3dB frequency of 23Hz. A decent turntable won't need that.
Ed
Ed
If the criterion is that the input transistor is to generate less noise than the resistors, it suffices to meet both of these conditions (assuming it's a decent low-noise transistor with reasonably low base resistance and 1/f noise, which seems to be the case):
- Keep the base current well below 1 uA to ensure base shot noise contributes less to the equivalent input noise current than thermal noise of R03 and R13 together
- Keep the collector current well above 13 uA to ensure that collector shot noise contributes less to the equivalent input noise voltage than thermal noise of R01.
Tr2 collector voltage needs to be centered between the supply voltage and the emitter voltage for optimized (~symmetric) clipping. You might expect that to be (28-1.25)/2= 13.375V but Tr2 does not have the current limitations that the 10K pull-up does so 11.5V is probably a good choice.
This is a classic phono preamp that was migrated from earlier tube circuits, and it has the same issues. The biggest issue is "harsh high frequencies" which is a result of the excessive loading that the RIAA network puts on the output. The output is capable of about (28-11.5)/10K= 1.65mA peak, which at high frequencies is driving 330 Ohms (R07) for a maximum voltage of 0.5445 Volts peak, about -4dBm. This may be good enough for a -20bBm input, but many find it unsatisfactory. The easy solution is an emitter follower 3rd transistor.
This is a classic phono preamp that was migrated from earlier tube circuits, and it has the same issues. The biggest issue is "harsh high frequencies" which is a result of the excessive loading that the RIAA network puts on the output. The output is capable of about (28-11.5)/10K= 1.65mA peak, which at high frequencies is driving 330 Ohms (R07) for a maximum voltage of 0.5445 Volts peak, about -4dBm. This may be good enough for a -20bBm input, but many find it unsatisfactory. The easy solution is an emitter follower 3rd transistor.
We're talking Phono preamps, output (of these vintage pre's) is some 150mVac; with 20dB overhead it needs 1.5Vac space within the dc-setting.
But that's right indeed, as it would improve performance considerable. The mindset in that time was to be 'conservative' in using resources, like expensive semiconductors in small metal cans. So, if not crucial, better omit.
But while modding the pre (op's goal), that would be the best option no discussion.
I remember the Valvo-Circuit (Year 1967):
All two-stage discrete circuits (Valve, Tube or BJT) are particularly fascinating. I think it's the many degrees of freedom and the simplicity that make them so special.
Unfortunately, the input resistance of the circuit is not constant over the entire audible frequency range. At the same time, the source resistance of our pickup, which is also frequency-dependent, interacts (often) with this input resistance and with the negative feedback loop.
And now it automatically becomes a challenge and is no longer quite as simple as it might seem. Statically, we can easily determine the operating points of the transistors, but beyond that it somehow becomes an art.
Well then,
I wish you every success with your experiments @percy
HBt.
All two-stage discrete circuits (Valve, Tube or BJT) are particularly fascinating. I think it's the many degrees of freedom and the simplicity that make them so special.
Unfortunately, the input resistance of the circuit is not constant over the entire audible frequency range. At the same time, the source resistance of our pickup, which is also frequency-dependent, interacts (often) with this input resistance and with the negative feedback loop.
And now it automatically becomes a challenge and is no longer quite as simple as it might seem. Statically, we can easily determine the operating points of the transistors, but beyond that it somehow becomes an art.
Well then,
I wish you every success with your experiments @percy
HBt.
Typical MM cart output is 5mv and preamp gain is 40db. So minimum 0.5v (500mvac) output. With +12db headroom for music peaks that would be 20mv in and 2Vac out. +15db would yield 2.8Vac out. So 3v would be safe margin.
I thought again about the simple two-stages with NPN transistors and dimensioned them step by step. With our current requirements or let's say simulation possibilities, I use this as a tool.
And here is my suggestion,
or even a universal template for other dimensioning ideas:
I concentrated on maximum operating point stability and symmetrical dynamic range, statically - in the first step ... VCE1 constant 3V, VCE2 constant 6V, IC1 225µA, IC2 2mA ..!
With BC 550 C or similar types, the circuit will deliver perfect results. Perhaps it will stimulate our imagination 🙂.
Greetings,
HBt.
And here is my suggestion,
or even a universal template for other dimensioning ideas:
I concentrated on maximum operating point stability and symmetrical dynamic range, statically - in the first step ... VCE1 constant 3V, VCE2 constant 6V, IC1 225µA, IC2 2mA ..!
With BC 550 C or similar types, the circuit will deliver perfect results. Perhaps it will stimulate our imagination 🙂.
Greetings,
HBt.
Looks like its playing some other role in addition to making the rumble filter. After I removed the sound drastically changed for the worse. High treble with increase in pops and ticks. I measured the capacitance of the pre and it had increased considerably - from ~100pf to ~400pf. The impedance also increased somewhat from ~48k to ~54k. So I think this capacitor is playing an important role in the cartridge loading as well. Before I put it back I am going to go the other way and try 0.2uf and see what happens.