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Old 6th November 2011, 12:05 PM   #1321
sonnya is offline sonnya  Denmark
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i am not a fan of current-thermal sensor. As when the current goes up and down, so does the FT of the BJT's
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Old 6th November 2011, 01:12 PM   #1322
Salas is offline Salas  Greece
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Quote:
Originally Posted by Lazy Cat View Post
Yes Salas, it was the PSRR debate but later on the topic was mainly concentrated to bias current thermal dependency and its cancellation. Current mirrors with middle current-thermal sensor is a pro-solution. I think LM134 linear response is adequate to maintain the currents in correct range to sustain stable working point conditions of the input pair.
Hi, can you point to a schematic with those in SSA front end? They are substituting the current feed resistors I suppose. Using a trimmer in one for adjustable current set? Any extra components? There is a 10mA max that will not allow much less than 100R emitter to ground at input transistors, yes?
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Old 6th November 2011, 02:57 PM   #1323
Sheldon is offline Sheldon  United States
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Thank you gentlemen. I think I had this almost correct, then came home last night after a wine tasting and tried to quickly finish up while rearranging for neatness. Hey, at least I didn't try to fly a jetliner. OK, I think I'm there. One question, how important is the low input impedance? My system can handle it, so just curious what we can get away with.

Sheldon
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Old 6th November 2011, 03:23 PM   #1324
Bigun is offline Bigun  Canada
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Sheldon, you have drawn the TGM5 version of SSA, more or less

Trouble with a low input impedance, in my case, is the use of a fairly high resistance passive (potentiometer) volume control before it is more challenging and of course, any caps you put in front need to be much larger or you lose bass response. As with TGM5 I suggest that you want to allow a provision for compensation caps on the CFP drivers as they like to oscillate if provoked (e.g at start up).
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Last edited by Bigun; 6th November 2011 at 03:25 PM.
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Old 6th November 2011, 03:36 PM   #1325
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Quote:
Originally Posted by Nico Ras View Post
Apassgear,

No disrespect intended, I always get a little concerned by power nuts. It is always a good measure to make the SSA as it was intended, familiarize yourself with it quirks, shortfalls and advantages, and most importantly get a feel for its sound whether it lives up to your expectation before pouring a bucket of transistors into it.

It is relatively easy to expand when you are satisfied that it is the amplifier that you want to live with.
Hi Nico,
I think this is the first time we "meet" at a thread, thanks for your advice and of course you are right. Even though I have built an Aleph 4 does not make me a power nut, what I'm after (as most of us) is the repro of the full audio BW at low to medium volume that suits one's speakers and of course with attributes such as the ones SSA has. From the amps I have built the FW F5 is near to what I need. Now that I'm writing this I realize that all the built amps have been with FET at the output, except for the tubed ones.
Maybe is time for me to try something different such as Lazy Cat's full BJT circuit shown on post 1319.
Cheers, Tony

Last edited by apassgear; 6th November 2011 at 03:42 PM.
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Old 6th November 2011, 03:37 PM   #1326
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Bigun View Post
Sheldon, you have drawn the TGM5 version of SSA, more or less
Indeed. I'm debating whether to build TC's simple thermaltrak version, or this one. I have the various devices left over from my FC 100 builds (ironically, the only active devices I don't already have in the bin are the common as dirt BC560).

Quote:
Originally Posted by Bigun View Post
Trouble with a low input impedance, in my case, is the use of a fairly high resistance passive (potentiometer) volume control before it is more challenging and of course, any caps you put in front need to be much larger or you lose bass response.
Didn't you trace some thermal stability problems to higher input impedance? I'd use this amp for the mid-highs in a multiamped system, driven by a DEQX, so low impedance is not an immediate issue. But for possible future application, it'd be nice to know the possibilities.

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Originally Posted by Bigun View Post
As with TGM5 I suggest that you want to allow a provision for compensation caps on the CFP drivers as they like to oscillate if provoked (e.g at start up).
No problem, as build will be on perf board.

Sheldon

Last edited by Sheldon; 6th November 2011 at 03:41 PM.
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Old 6th November 2011, 04:08 PM   #1327
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Sheldon View Post
Indeed. I'm debating whether to build TC's simple thermaltrak version, or this one. I have the various devices left over from my FC 100 builds (ironically, the only active devices I don't already have in the bin are the common as dirt BC560).
I would suggest you use the Thermaltrak's that you have - good use of good parts. A Thermaltrak TGM5 !

Quote:
Originally Posted by Sheldon View Post
Didn't you trace some thermal stability problems to higher input impedance? I'd use this amp for the mid-highs in a multiamped system, driven by a DEQX, so low impedance is not an immediate issue. But for possible future application, it'd be nice to know the possibilities.
Yes, I did, but in simulations only - haven't had a chance to build it yet. I think there are diminishing returns below 10k or so because the low impedance helps only with the offset due to mismatch of input devices - it doesn't help if they aren't well thermally coupled and it does nothing to help imbalance caused by temperature differences in the VAS devices. The latter is something I'm not too worried about but it's another factor.
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Old 6th November 2011, 06:53 PM   #1328
Sheldon is offline Sheldon  United States
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Originally Posted by Bigun View Post
I would suggest you use the Thermaltrak's that you have - good use of good parts. A Thermaltrak TGM5 !
That's a given. Decision is whether to use simple version or CFP version.

Sheldon
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Old 6th November 2011, 08:06 PM   #1329
Bigun is offline Bigun  Canada
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I like the CFP because I believe the Triple better isolates the VAS from the non-linear impedance of the output devices. Everyone has their own preferences I guess - Hugh prefers a Triple based on a Diamond Buffer, OStripper prefers a Triple based on cascaded EFs.

The CFP hates Class B, so in the driver position it must be kept in Class A with sufficient current flow. I like that it doesn't rob me of any voltage headroom compared with a Triple based on Cascaded EFs and that it doesn't need current sources like a Triple based on Diamond buffer (I don't like current sources or current mirrors).
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Old 7th November 2011, 04:19 AM   #1330
Ruwe is offline Ruwe  Europe
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Hi,
I see the discussion is going in many different directions since last time when I checked, but I hope some of you may find my last findings useful.

In my previous post #937
Simple Symetrical Amplifier I had various problems with Lazy Cat's SSA. I know most of you are trying more complex schematics derived from the simple one, but I am too stubborn or just silly, and wanted to make the original simple one to pass my usual amplifier tests. Finally, I had to discard the use of vertical MOSFETS and BJTs, because of their high temperature drift. I can assure you that I've spent hundreds of hours (I'm not exaggerating) and tried many output configurations. None of them had stable quiescent current and no modification or change in the schematic values gave me reliable results.

So, I bought laterals BUZ906P and BUZ901P. With them I could go back to the original schematic with just a trimmer for current adjustment. These transistors have their own limitations, one of them is that they are not very well matched N to P channel at low currents. Another one is low gain (transconductance). Third one is that they work better with higher idle current (150mA is the reasonable minimum).

After many tests I attach here my version of the schematic. I used the drawing of the forum member juma as a template.

With the new values I solved some additional problems:
1. Circuit gain was not constant with the original values in the feedback. It was dependent on the output load and the output amplitude. The amp behaved like one with high output impedance, but mostly due to circuit gain change. I noticed variations from gain of 10.5 to 7. Lower feedback resistors values solved that.
2. I tried to balance the laterals for minimum distortion. See the change in their gates. Of course its just in the ballpark. It's impossible to match for all Vgs/current values.
3. My SSA version has 4MHz oscillation that tends to show itself with some speakers and cables. I added capacitors for stability. Values depend on how brave are you and how much risk you would accept. I am not brave, but you can even run it without capacitors.
4. In the original SSA, with one channel loaded at half power, I noticed that the other channel changed its otput DC component from 10mV to, say, 200mV. After investigation I found it was due to insufficient current in the Zeners. When the supply drops several volts, the current in zener diodes goes close to the "knee" value and the regulation is lost. So, I increased significantly the Zener diodes current. Have in mind that much of it is robbed trough R12/R13 and R11/R14. As Lazy Cat suggests, regulated supplies are good option for this amp.
5. With the new values the voltage in the nodes R12-R13-R7 (resp. R11-R14-R10) was insufficient to put into conduction Q5 and Q6, so I had no choice but to increase the Zener voltage with 18V diodes.
6. Due to the higher currents you'll need 0.5W or 1W resistors as marked on the schematic.
7. The laterals like more current. If you run them at less than 150mA, there is prevailing 3rd harmonic. It will completely disappear at higher idle current. The idle current is stable about 10 minutes after turn-in. DC component is stable too. It will change from about +/- 10mV to about 100mV at half power. If you adjust the laterals for more idle current (say 250mA), the DC component will not change to more than 30mV at half power.
8. I like capacitor at the input. IMO, it solves more problems than it creates.

That's all so far. The amp sound is very good, but I'll give it more listening once I build decent boards and put it in an enclosure. The original version sounds kind of too warm and one-note in the bass to my ears. My definition for good sound is detail and definition, but not on the "clinical" side. The type of 300B single-ended comes to mind. I think this circuit has the potential to be close.
If you are interested, Nelson Passes' F5 article (manual) and his Super Symmetry article are very interesting reading.

Regards
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Last edited by Ruwe; 7th November 2011 at 04:29 AM.
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