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Mods to "Little Bear" phono-preamp

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#63
ejp said:
No. What I said is that it is inherently inaccurate. I haven’t said anything about stability. You keep putting words into my ,out.
Click to expand...

It simulates (in LTSpice) and measures very accurately. Input and output spectrum and real-time live signals match exceptionally well too (at least for iterations of the basic circuit which I have further developed).

I am only asking questions to most respectfully better understand why there is a discrepancy between accurate measured results and algebra which says it will be inherently inaccurate. Of course, maybe the inaccuracies we are discussing are potentially very small and thus not significant enough to be accurately measured with my equipment.

I do not mean any disrespect, and wish to convey this most sincerely. 🙂
 
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#64
rongon said:
I don't know whether or not the schematic is an accurate representation of the PCB layout. Assuming it is...

View attachment 1342971

That looks like pin 9 is grounded to 0V signal ground. I see no voltage divider for lifting the ground of the heater supply. I think that's referenced to 0V.

The schematic for the heater supply...
View attachment 1342974

...shows a "15V - 15V" winding that is not center tapped. What does that mean? I don't know. I assume it's a 15VAC secondary. Assuming that's correct, D1 will rectify that to about 19V DC. It looks like that's grounded on the positive side, so that means it's a -19V DC supply, with the voltage drop from 10R 5W (R26) subtracted. With three 12A*7 tubes' heaters as the load, that's 0,45A, so 19V - 4.5V = 14.5V DC on the heaters? Maybe there are more losses than that. It could be that the raw voltage is only 17V so the final DC voltage comes out to -12.5V. Let's assume that's correct.

1. The DC heater supply is grounded on one side. No center tap. That will increase the chances of hum from the heater supply. Since it's grounded on the DC side, you do not want to ground it again on the AC side, so you can't put in a voltage divider to balance the secondary.

2. There's not a lot of ripple filtering on the heater supply. 10 ohms * 4700uF, and just one stage of RC after the reservoir cap.

3. It could be that the hum is coming from the heater supply. I've never had good luck with a simple rectified DC supply like that. I either go with AC heaters or a decent regulator (LM317, 7812, LT1085, etc.). Unfortunately, the designers of this product spent their money on a 10R 5W resistor instead of a voltage regulator IC. Wouldn't that have cost about the same?

Maybe increasing the value of C2 would help. It could be 10,000uF 25V. But that might not be enough. I have my doubts, but I certainly don't know for sure.
Click to expand...
So, what options could I use to alleviate the problem other than a larger capacitor?
 
#65
soulmerchant said:
It simulates (in LTSpice) and measures very accurately. Input and output spectrum and real-time live signals match exceptionally well too (at least for iterations of the basic circuit which I have further developed).

I am only asking questions to most respectfully better understand why there is a discrepancy between accurate measured results and algebra which says it will be inherently inaccurate.
Click to expand...
And for me, what matters even more is what does something sound like. I've tried doing mods to an amp that folks said LT spice or other math says would make it better, but in practice the amp sounded worse in person. This EAR834 circuit sounds very good and people wouldn't be cloning it decades later if it didn't.
 
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#67
This topo is a favorite for cartridges less sensitive to C loading, like the Denon DL160 and DL110, etc. but Miller C of the first stage is a challenge for high inductance cartridges. It could be described as a transconductance first stage and an anode follower second stage, named by C. P. Boegli back in the Kennedy era. MarcelvdG described a modern version designed to allow AC heating in LA vol4.

One could argue that any practical valve phono stage will always be starved for open loop gain, and so will always approach running out of loop gain, so why not let it run to open loop below 50Hz?

All good fortune,
Chris
 
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#68
stephe said:
And for me, what matters even more is what does something sound like. I've tried doing mods to an amp that folks said LT spice or other math says would make it better, but in practice the amp sounded worse in person. This EAR834 circuit sounds very good and people wouldn't be cloning it decades later if it didn't.
Click to expand...

It sounds much better to me with the mods I made. Especially replacing the 12ax7 cathode follower with a mosfet follower circuit, regulated power supply, higher B+ (for higher plate voltage on first 12ax7) - but in it's most budget form it is still pretty darn good.
 
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#69
ejp said:
No it isn’t. Compute the passive loss at 20kHz.
Click to expand...
Did you know that this circuit makes use of the plate resistance of the input 12ax7 to passively attenuate higher frequencies as well?

If you re-engineer the circuit to have greater plate voltage on the entry 12ax7 (as I have done) then pretty much every 12ax7 will sound the same (exceptionally good). The circuit is quite clever in its use of the 12ax7 plate resistance as a passive component for RIAA correction. Tim de Paravancini knew this and made use of it.

Further attenuation of higher frequencies to accurately match RIAA is of course accomplished by the feedback loop.

It's a simple looking yet quite clever circuit. I didn't fully appreciate it until I attempted designing my own RIAA amplifiers using other high-gain triodes and pentodes...

If you swap the 12ax7 with different high-gain triodes (with similar high mu and lower plate resistance), you need to completely re-engineer the circuit for accurate RIAA, and they invariably need to deliver a much greater correction (attenuative effect) too!

The circuit certainly makes use of passive elements for RIAA correction. They are just not the elements we might usually consider.
 
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#70
rongon said:
I don't know whether or not the schematic is an accurate representation of the PCB layout. Assuming it is...

View attachment 1342971

That looks like pin 9 is grounded to 0V signal ground. I see no voltage divider for lifting the ground of the heater supply. I think that's referenced to 0V.

The schematic for the heater supply...
View attachment 1342974

...shows a "15V - 15V" winding that is not center tapped. What does that mean? I don't know. I assume it's a 15VAC secondary. Assuming that's correct, D1 will rectify that to about 19V DC. It looks like that's grounded on the positive side, so that means it's a -19V DC supply, with the voltage drop from 10R 5W (R26) subtracted. With three 12A*7 tubes' heaters as the load, that's 0,45A, so 19V - 4.5V = 14.5V DC on the heaters? Maybe there are more losses than that. It could be that the raw voltage is only 17V so the final DC voltage comes out to -12.5V. Let's assume that's correct.

1. The DC heater supply is grounded on one side. No center tap. That will increase the chances of hum from the heater supply. Since it's grounded on the DC side, you do not want to ground it again on the AC side, so you can't put in a voltage divider to balance the secondary.

2. There's not a lot of ripple filtering on the heater supply. 10 ohms * 4700uF, and just one stage of RC after the reservoir cap.

3. It could be that the hum is coming from the heater supply. I've never had good luck with a simple rectified DC supply like that. I either go with AC heaters or a decent regulator (LM317, 7812, LT1085, etc.). Unfortunately, the designers of this product spent their money on a 10R 5W resistor instead of a voltage regulator IC. Wouldn't that have cost about the same?

Maybe increasing the value of C2 would help. It could be 10,000uF 25V. But that might not be enough. I have my doubts, but I certainly don't know for sure.
Click to expand...
Do you think a 10000uf or 22000uf capacitor in C2s position would possibly help the poor filtering?
 
#71
Yes, more capacitance would filter more 60Hz and 120Hz hum from the heater supply. But would it be enough? I don't know.
As I wrote, I've never had a good experience trying to get a quiet DC heater supply without regulating it.

What is the DC voltage at the negative terminal of C1 (in the power supply schematic)?
What is the DC voltage at VDD?
 
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#73
Since I already simulated EAR834P in Multisim I couldn't resist to for Little Bear too.

First picture Little Bear 1,4dB diff
Second picture EAR834P original 0,8dB diff
Third a linearized EAR834P 0,2dB diff

Skärmbild 2024-08-11 223224.jpg


Skärmbild 2024-08-11 223613.jpg


Skärmbild 2024-08-11 224416.jpg
 
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#74
Just for comparison...

First picture: Audio Research SP6C (7,8dB down to standard RIAA, diff 0,3dB)
Second picture Audio Research SP8 (5dB down to standard RIAA, diff 0,7dB)
Third picture: Audio Research SP10C (2,8dB up to standard RIAA, diff 0,5dB)

SP10C are here simulated with half the count of tubes and RIAA network adjusted accordingly.
SP10 have parallelled tubes in all places.

SP6C.jpg


SP8.jpg


SP10revC.jpg
 
#75
rongon said:
Yes, more capacitance would filter more 60Hz and 120Hz hum from the heater supply. But would it be enough? I don't know.
As I wrote, I've never had a good experience trying to get a quiet DC heater supply without regulating it.

What is the DC voltage at the negative terminal of C1 (in the power supply schematic)?
What is the DC voltage at VDD?
Click to expand...
The measured voltage at C1 was 18v. All three tubes measured about 6.03 v. I got impatient and replaced C2 with a 10,000 uf capacitor. This dropped the dc voltage to around 17.8v. All three tubes measured about 5.95v. This did drop the amount of hum considerably. I’m going to connect to my turntable and amp sometime today.
 
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#80
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