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Old 12th March 2019, 11:02 AM   #11
jan.didden is offline jan.didden  Europe
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Are you expecting us to derive a circuit from a PCB layout and then comment on the used tubes??

Jan
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Old 12th March 2019, 11:45 AM   #12
dadod is offline dadod  Croatia
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Quote:
Originally Posted by jan.didden View Post
Are you expecting us to derive a circuit from a PCB layout and then comment on the used tubes??

Jan
I think he means transistors.
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Old 12th March 2019, 12:13 PM   #13
FengLiu is offline FengLiu
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I think he means transistors.
Yes, I mean a transistor. The transistor type of the original circuit is too old. Can it be replaced by modern transistors?
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Old 12th March 2019, 12:38 PM   #14
dadod is offline dadod  Croatia
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Quote:
Originally Posted by FengLiu View Post
Yes, I mean a transistor. The transistor type of the original circuit is too old. Can it be replaced by modern transistors?
You can replaced with new modern but you have to know what to use.
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Old 13th March 2019, 01:00 PM   #15
sgrossklass is offline sgrossklass  Germany
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All the transistors on this board are still available AFAICT. It would probably be wise to replace the commonly counterfeited and long EOL 2SC3281/2SA1302 with the in-production MJL3281/1302 or similar. MJE340/350 are still active. MPSA06/56 also are, but do not seem to be made in large numbers, making them a bit more expensive than some other TO-92s (2N5550 may be a suitable but somewhat less good replacement).
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Old 13th March 2019, 04:04 PM   #16
Ylli is offline Ylli  United States
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Question on a typical LTP (Schematic snip from a Heathkit AR-1500).

It can be seen that the total LTP emitter current is nominally 4 mA. Yet, with a 680 ohm collector resistor, the static current in the Q1 side will settle in at about 1 mA, leaving 3 mA for the Q2 side.

This is obviously an unbalanced condition. It seems the general consensus that matching the input devices is important for lowest distortion and to minimize output offset. With the static imbalance apparently 'designed in', there is no way one is going to get the output offset down to a reasonable value without injecting current into one of the inputs from an outside source.

I've seen this in amplifiers other than just the Heathkit. Why are amplifiers sometimes designed this way with the intrinsic imbalance?
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Old 13th March 2019, 10:49 PM   #17
Marc Vi is offline Marc Vi  Netherlands
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I think you are wrong. Through Q1 and Q2 goes the same current of 2 mA each. Through R4 goes 1 mA and the other 1 mA goes from the base of Q3 to the emittor of Q3.
But I agree this is not a good design.
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Old 13th March 2019, 11:03 PM   #18
Ylli is offline Ylli  United States
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Didn't show the full schematic, but the collector current source for the VAS is less than 10 mA. Assuming a reasonable hfe of 50 for the VAS, that means it needs 0.2 mA of base current.

So the LTP has 1.2 mA in the Q1 branch, and 2.8 mA in the Q2 branch. Still considerably unbalanced by design.

Last edited by Ylli; 13th March 2019 at 11:11 PM.
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Old 13th March 2019, 11:20 PM   #19
Marc Vi is offline Marc Vi  Netherlands
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Indeed it needs at least 0,2 mA of base current but the base current can be much higher even with a collector current of 0,2 mA. The base-emittor of Q3 functions more as a diode.
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Old 14th March 2019, 05:38 AM   #20
FengLiu is offline FengLiu
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Quote:
Originally Posted by Ylli View Post
Question on a typical LTP (Schematic snip from a Heathkit AR-1500).

It can be seen that the total LTP emitter current is nominally 4 mA. Yet, with a 680 ohm collector resistor, the static current in the Q1 side will settle in at about 1 mA, leaving 3 mA for the Q2 side.

This is obviously an unbalanced condition. It seems the general consensus that matching the input devices is important for lowest distortion and to minimize output offset. With the static imbalance apparently 'designed in', there is no way one is going to get the output offset down to a reasonable value without injecting current into one of the inputs from an outside source.

I've seen this in amplifiers other than just the Heathkit. Why are amplifiers sometimes designed this way with the intrinsic imbalance?
Where is your circuit diagram coming from?
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