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Old 12th September 2006, 02:35 AM   #1431
pooge is offline pooge  United States
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Quote:
Originally posted by AndrewT
Hi Pooge,

Out of interest I tried a few more output offset measurements and sure enough the mixed coupling causes excessive output offset.
Here are my measurments with r17=33k, all else as Jens' Bom.

a. input of DC blocking cap to ground, offset=-2.6mV (AC coupled at input and NFB, measurement done cold and will drop to about -1.4mV when fully warmed up)
b. output of DC blocking cap to ground, offset=72.7mV (Leach original connections)
c. output of DC blocking cap thro' 51r to ground, offset= 72.1mV (mimics a 51r source impedance)


But you haven't tried output of DC blocking cap left alone, i.e., without connecting it to ground or another resistor. (Actually, this will probably give you the same offset as "a".) I don't see how you ruled out admittedly unlikely DC leakage throug the DC blocking capacitor. Measure offset with and without the input of the DC blocking capacitor connected to ground. Leave the other side alone.
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Old 12th September 2006, 02:55 AM   #1432
pooge is offline pooge  United States
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Quote:
Originally posted by AndrewT
Hi Pooge,

I have found great consistency in output offset between my two Leach amplifiers using the input resistors Bommed by Jens. But the DC offset is not nor near zero. Both amplifiers need r17 to be adjusted upwards to about 33k to reduce DC output offset to near zero. This offset remains very close to zero over long term testing. It moves slightly but holds there if the PSU supply voltage is changed, even though the Iq changes dramatically causing significantly different temperatures in the output stage.
33k is some 50% higher than 22k.
Why should this be necessary?
Actually, your DC offset was fine for unmatched transistors.
The drift you see may be more likely related to temperature drifts in the input pair. Put a finger on one of them and notice the drift! Keeping the input pair to zero temperature differential is very desirable for preventing drift. This is why I was looking for a suitable matched pair dual. Alternatively, bonding the pair together to keep them in thermal equalibrium is a suitable option. It would be nice to have a board layout that made this easy. I'm wondering if there's a specialized heat sink out there that will clamp the pair together for this purpose without requiring Krazy Glue.
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Old 12th September 2006, 03:35 AM   #1433
pooge is offline pooge  United States
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Quote:
Originally posted by AndrewT

If one wants to operate the Leach as a DC coupled amplifier, then one needs to short out the DC blocker on the input HIpass filter and short out the BC blocking cap in the NFB path (lower leg). When you do both then the output offset should remain low or near zero. I have not measured either of my amplifiers for this condition. What I have done in my last posting was to point out that if one partially converts the Leach to DC coupled by only shorting out one of the two DC blocking capacitors one will find it very difficult to arrange for the input impedances to be set to minimise the output offset to an acceptably low level and still retain sensible impedances for the two inputs.

This becomes apparent if you do a fully DC coupled Leach and then attach a DC blocked pre-amp (most are) to the input RCA. DC output offset will go high.
The solution is to use Jens' alternative input which he has on board. DC or AC coupled input with two RCAs at the chassis for connecting the appropriate type of pre-amp.

I don't know why you keep worrying about "impedances", which is important under AC conditions, when you are trying to analyze DC conditions, where resistances are what matters. I outlined the DC resistance paths in a previous post, and you keep insisting on shorting these out, adding another one in parallel, or whatever. DC balance is just that. AC is not involved.

If you short the input of an amp with a series input cap, it shouldn't affect DC offset. If you short the amp with no blocking cap, then you essentially short the input grounding capacitor and the balance will be upset. This is why you should measure offset without the input shorted, unless you have a blocking capacitor.

If you have a pre-amp with a DC blocking capacitor in the output, and your amp offset changes when connected to the preamp, then the preamp likely has a resistor to ground at the output after the cap that is placed in parallel with the input grounded resistor in the amp, thus upsetting balance.

So a designer can't design the amp to be perfectly matched with every component. That's why some use an offset trim pot in the input stage. You can reduce the offset by changing the resistor as you have, but there is also the noise factor you have to juggle, as the input transistors have their lowest noise with a particular source impedance, which may not be the lowest impedance.

Self found the highest sensitivity of offset was from base currents. These base currents generate a voltage in the input resistor path and the feedback resistor path. It is these voltages arising out of the base currents which causes the major offset sensitivity. This sensitivity is reduced by lowering the resistance values to lower the magnitude of the base current induced voltages, to thereby reduce the magnitude of variations, and/or use high beta transistors, to reduce the magnitude of the base currents. It is a matched beta that matches the base currents. Of course, matching of the input degeneration resistors helps reduce offset from this source, which Self indicates is not likely as high an error.

So it is not so much that the preamp is DC blocked. It is the paralleling of other resistors to the input resistors (or the shorting of these resistors) that upsets the voltage balance of even equal base currents from a matched transistor pair.
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Old 12th September 2006, 07:11 AM   #1434
loek is offline loek  Netherlands
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Default a specialized heat sink

Hi Pooge, if you watch the TO-5 transistors in my last picture you sea the star-type heatsinks.(spring effect)
They also fit on two tied TO-92 type transistors.
Don't know if they still are available. I will search for them.
You can also use heatshrinkable tube.
Greetings, Loek
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Old 12th September 2006, 07:26 AM   #1435
AndrewT is offline AndrewT  Scotland
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Hi Pooge,
Quote:
But you haven't tried output of DC blocking cap left alone, i.e., without connecting it to ground or another resistor.
actually I did way back to check what effect an open input would have
Quote:
to ensure that the open circuit that otherwise be there cannot cause an input instabilities that may cascade down stream (although I think Jens' interpretation of Leach does not suffer from this input instability problem).
A basic check, but it does not provide much useful evidence on DC conditions nor noise performance, since a low source impedance is usually connected.
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Old 12th September 2006, 01:02 PM   #1436
pooge is offline pooge  United States
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Default Re: a specialized heat sink

Quote:
Originally posted by loek
Hi Pooge, if you watch the TO-5 transistors in my last picture you sea the star-type heatsinks.(spring effect)
They also fit on two tied TO-92 type transistors.
Don't know if they still are available. I will search for them.
You can also use heatshrinkable tube.
Greetings, Loek

Wow! That picture reminds me of my first Leach with the ground plane. I made the board myself. What a pain that was, to line up everything on both sides of the board without all the modern tools. I couldn't believe it when it worked the first time. It worked for many years until I did something stupid by pulling RCAs while it was on. I was ready to upgrade, anyway.

I have some of the star heat sinks. I'll have to look into that possibility. Don't know if I'd use heat shrink, though. Even though they are not hot transistors, I wouldn't choose to insulate around them. I think it would be better to bond the flat sides together with adhesive, preferably thermally conductive. (Anyone know of any thermally conductive adhesive in a small tube?) Haven't studied the lead twisting issue on this one, because I haven't settled on transistor selection yet.
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Old 12th September 2006, 01:20 PM   #1437
mlloyd1 is offline mlloyd1  United States
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in addition to using the mentioned "star heat sink" for keepinjg TO92 parts somewhat in thermal equilibrium, i have also wrapped both parts together in a loop or two of thick copper wire with a VERY small amount of heat sink grease between them.

good luck!
mlloyd1
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Old 12th September 2006, 02:53 PM   #1438
AndrewT is offline AndrewT  Scotland
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Hi,
sometimes you can turn the pair of To92 through 90 degrees to face flat sides together. Then bond them with instant glue. Very thin=low thermal resistance.

I have not tried this on the Jen's board but it worked on the GB150 SKA pcb. Leave the To92 legs a bit longer and add very thin insulation (stripped from 0.6mm wire) may give sufficient flexibilty to achieve this.
Four turns of thick copper wire around the pair with longish tails gives a bit of extra dissipation capacity.
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Old 2nd November 2006, 02:04 PM   #1439
pooge is offline pooge  United States
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Been re-reading old amplifier papers since Cordell's entry into this forum, in preparation for building this amp, and I'm have a major brain dump. Leach says his amplifier has a GBP of 8.5 MHz, while his specified output transistors have an ft of 2MHz. I thought that ft was the maximum bandwidth, i.e., at a gain of unity. Therefore, how you can get a GBP of 8.5 is not registering with me right now. What am I missing?
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Old 2nd November 2006, 02:45 PM   #1440
AndrewT is offline AndrewT  Scotland
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Hi,
too good a question.

I could be wrong, so we'll be waiting for a correct explanation, but here's mine.

The fT of the output device is for when it is producing gain, however Leach uses it as a follower (no gain, well just less than 1.0).
The preceding circuit produces all the gain.
The combination of the follower and the voltage amplifier has a GBW product of 8.5MHz, so it's this combination that has a gain of 1 @ 8.5MHz and rises @ 20db/decade as frequency falls. i.e. gain =100 @ 85kHz.

Finally, the open loop gain is corrected by wrapping the NFB loop around the whole thing to reduce the closed loop gain to 23times (22k/1k + 1=23).
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