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Old 9th July 2014, 09:47 PM   #1
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Lightbulb lGl-2, continuing "hybrid madness" - no GNFB class A

Hello All,

Based on successful prototyping of the first hybrid with tube-based LTP input stage, I'm moving forward, trying to utilize the advantages of "both worlds". Tube-based front-end, performing all the voltage gain, and solid-state power section (class A unity voltage gain buffer), designed by Damir (the best one I'm aware of), performing all the current gain. Trying to keep the whole thing as simple as possible.

Here are some initial simulation results. Look very promising.
Voltage gain is set to standard 29db
OPS quiescent current is 2.2A
Attached pictures show the schematic and some simulations.

First option is purely NO GNFB - only some cathode degeneration. Tubes at their best.
Second option - same schematic with 2.8db gain increase at the second stage and then dropping the same amount with GNFB. Slightly reduced distortion, however - not too much difference, so... just as an experiment. Attempts to further increase the open loop gain and compensate it by GNFB result in overall distortion increase (12au7 is a relatively low gain tube, high linearity though).

I really like very low THD increase between 1KHz and 20KHz at all levels.

Any thoughts, comments, ideas will be greatly appreciated.

Cheers,
Valery
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Old 10th July 2014, 03:00 PM   #2
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Default Decided - no GNFB

Here is an updated schematic - already very close to what I'm going to prototype...
Attached Images
File Type: jpg @LinearGain-Hybrid-01-NoGNFB-01-sch.jpg (256.2 KB, 362 views)
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Old 10th July 2014, 04:53 PM   #3
Bigun is offline Bigun  Canada
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Looks very good to me. This OPS looks like a good candidate for others to use in their own hybrid plans - so we will be keen to know how well it works for you here in terms of stability and listening.
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Old 11th July 2014, 05:25 AM   #4
Calvin is offline Calvin  Germany
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Hi,

I wondered about the RC-lowpass (100R/150p) at the Szikay´s Master transistor Bases (Q3,4).
I rather know the compensation as single cap or as RC from the Collector to the Base of the Slave transistor (Q5,6) or in parallel to the Master´s collector resistor (R14,17).
Is there an advantage using the bandwidth limiting RC at the Bases, instead of local (or internal) compensation?
What were the parameters for choosing the VD139/140 and MJL1302A/3281A?
They are rather slow and the MJLs not low capacitance devices.
This might lead to oscillation of the Sziklay pair at rather lowish frequencies)
R14/17 are fortunately very low in value, but still low-cap power transistors might be a good alternative, for example 2SA1186/SC2387 (Sanken, ISC).
The 2SB649/2SD669 driver transistors can drive a pair of 1186/2837 each (R14,17=150R, added 0R22 Emitter resistors for 1186/2837 and R3,9=0R22)

jauu
Calvin

Last edited by Calvin; 11th July 2014 at 05:43 AM.
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Old 11th July 2014, 10:07 AM   #5
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Hi Calvin,

As I mentioned in the first post, the output buffer is actually designed by Damir - I just slightly modified it, adding second output pair. 100R/150p are acrually a base stopper and shunt compensation capacitor, forming a roughly 10MHz low pass filter, as you mentioned, ensuring stability of Sziklai pairs. This choice of transistors was Damir's choice, however I have simulated the whole thing and it shows excellent performance, including the bandwidth and level of distortion. I also know that this buffer was built by Damir and some other people here with very good impressions.

I also like Sankens and use them frequently, however in this case I just use a successful design. My major interest in this project is to test the sound of tube-based voltage gain stage with no global NFB.

Cheers,
Valery
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Old 11th July 2014, 11:40 AM   #6
dadod is online now dadod  Croatia
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Quote:
Originally Posted by vzaichenko View Post
Hi Calvin,

As I mentioned in the first post, the output buffer is actually designed by Damir - I just slightly modified it, adding second output pair. 100R/150p are acrually a base stopper and shunt compensation capacitor, forming a roughly 10MHz low pass filter, as you mentioned, ensuring stability of Sziklai pairs. This choice of transistors was Damir's choice, however I have simulated the whole thing and it shows excellent performance, including the bandwidth and level of distortion. I also know that this buffer was built by Damir and some other people here with very good impressions.

I also like Sankens and use them frequently, however in this case I just use a successful design. My major interest in this project is to test the sound of tube-based voltage gain stage with no global NFB.

Cheers,
Valery
Valery, you can take a look at my last schematic in GainWire-CLASS-A power amp (I am out of my house and using tablet to sent this answer and dont know how to copy the link with it) and see that I used the diamond instead Baxandall super pair as Elwee suggested, more stable with CFP.
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Old 11th July 2014, 01:58 PM   #7
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Quote:
Originally Posted by dadod View Post
Valery, you can take a look at my last schematic in GainWire-CLASS-A power amp (I am out of my house and using tablet to sent this answer and dont know how to copy the link with it) and see that I used the diamond instead Baxandall super pair as Elwee suggested, more stable with CFP.
Hi Damir!

Do you mean the one with diamond and bootstrap from the post #60 there?

Thank you,
Valery
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Old 11th July 2014, 02:08 PM   #8
dadod is online now dadod  Croatia
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Quote:
Originally Posted by vzaichenko View Post
Hi Damir!

Do you mean the one with diamond and bootstrap from the post #60 there?

Thank you,
Valery
Hi Valery, Yes it is the post #60, but you dont need to use bootstrap, try to simulate with the square wave.
Damit
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Old 11th July 2014, 04:09 PM   #9
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Quote:
Originally Posted by dadod View Post
Hi Valery, Yes it is the post #60, but you dont need to use bootstrap, try to simulate with the square wave.
Damit
Damir, you're right - this way it shows better stability. I have also decreased the base stoppers value to 10 ohms and shunt caps to 47pF, as you suggest in the latest schematic, and it actually stays stable even without the caps, being close to margin though. With 47pF caps in place it looks excellent.
Also changed the CCS output transistors to 2N5401/5551 as with my +/-36V rails BC550/560 run out of SOA in case of high output swing.

Started preparing for prototyping. I will build the front-end and the buffer at separate PCBs, so that I can experiment with different combinations. This is going to be interesting

Cheers,
Valery
Attached Images
File Type: jpg @LinearGain-Hybrid-01-NoGNFB-02-sch.jpg (152.4 KB, 335 views)
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Old 13th July 2014, 11:30 AM   #10
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Quote:
Originally Posted by Calvin View Post
Hi,

I wondered about the RC-lowpass (100R/150p) at the Szikay´s Master transistor Bases (Q3,4).
I rather know the compensation as single cap or as RC from the Collector to the Base of the Slave transistor (Q5,6) or in parallel to the Master´s collector resistor (R14,17).
Is there an advantage using the bandwidth limiting RC at the Bases, instead of local (or internal) compensation?
What were the parameters for choosing the VD139/140 and MJL1302A/3281A?
They are rather slow and the MJLs not low capacitance devices.
This might lead to oscillation of the Sziklay pair at rather lowish frequencies)
R14/17 are fortunately very low in value, but still low-cap power transistors might be a good alternative, for example 2SA1186/SC2387 (Sanken, ISC).
The 2SB649/2SD669 driver transistors can drive a pair of 1186/2837 each (R14,17=150R, added 0R22 Emitter resistors for 1186/2837 and R3,9=0R22)

jauu
Calvin
Hi

Where do you found 1186 ?

I would like made a CFP power buffer
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