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Old 13th September 2012, 11:38 AM   #871
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Hi Dado
If you feed V+ and V- in at the middle of the output transistors as Andrew T and Harry recommend then you can run the feeds to the collectors out and return the current from the emitters back on the other layer directly on top. This is what I recommended to Harry earlier - I don't see how the loop can be made any smaller than this, it is effectively only the thickness of the board.

As to the feedback circuit. It occurs that it would be possible to send the trace (or wire) from the centre point of the output transistors back to the input then have the feedback divider resistors there, with the earth return back to the earth point between the output transistors. This should cancel any inductive couple and not dump the feedback current into the quiet earth.

Best wishes
David
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Old 13th September 2012, 11:57 AM   #872
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Originally Posted by keantoken View Post
In a high-performance amp there [w]ill always be nodes of high impedance. The usual technique is to use indefinite or infinitesimal impedances to increase gain. Therefore even slight capacitive coupling from the output in the [w]rong place [w]ill certainly cause oscillation
That is a return to the topic of this thread because it is my principle concern with Edmond's circuit. The hi gain is produced by the use of very hi impedance loads so I expect it will be fairly sensitive to stray capacitance and the like. I would like to use more current gain at lower impedance levels for a similar loop gain result. Not sure about how best to do that.
In ICs they scale the area of transistors to have current mirrors where the current in one path is a multiple of the other.
Anyone know if that would work here?

Best *ishes
David

BT* I think you try to use *ords *ith * so it *ill look funny
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Old 13th September 2012, 01:05 PM   #873
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Quote:
Originally Posted by Dave Zan View Post
... I would like to use more current gain at lower impedance levels for a similar loop gain result. Not sure about how best to do that.
In ICs they scale the area of transistors to have current mirrors where the current in one path is a multiple of the other.
Anyone know if that would work here?

Best *ishes
David
I have a current feedback design where for modest current increase (2-4) I use parallel transistors, also if available one can use different degeneration resistor values. Each of these come with their own drawbacks but nothing is free (except for the letter w, wwww)

Thanks
-Antonio
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Old 13th September 2012, 01:52 PM   #874
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Dave Zan View Post
.................As to the feedback circuit. It occurs that it would be possible to send the trace (or wire) from the centre point of the output transistors back to the input then have the feedback divider resistors there, with the earth return back to the earth point between the output transistors. This should cancel any inductive couple and not dump the feedback current into the quiet earth............
Don't take the NFB back to the Power Ground.

The +IN & -IN pins of the LTP (or other inputs) measure the Signal.

That signal comes in via the chassis located input socket (RCA, DIN XLR etc.).
The signal hot and the signal return are the two points/inputs about which the amplifier tries reproduce a copy at it's own output.
Give the amplifier the best chance it has of reproducing the input signal by connecting the input socket to BOTH of the amp inputs. That requires one to couple the NFB lower leg to the signal return, not to Power Ground.
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Old 13th September 2012, 03:36 PM   #875
dadod is offline dadod  Croatia
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Originally Posted by HarryDymond View Post
Hi Dado,

You may have spotted earlier that I mentioned that layout issues hurt my brain more than I would like. Figuring out if you have a problem is usually not hard*; the head-hurty bit is trying to figure out how to solve the problem! The bad news is that with your TT amp layout, I have done (some of) the easy bit in that I can confirm there are issues with it. On the image below, I have traced the current pathway for current from the +ve decoupling capacitor. Note how the loop area is large and that the loop partly encloses the front-end of the amplifier; not good!

Click the image to open in full size.

Now you've got to do the hard bit, which is figure out how to make those issues go away. Have another look at my "improved layout" image which hopefully will help; start by bringing the power connection in at the horizontal midpoint between the PNP and NPN devices at the top of your board, together with the ground.

* To see if there are problems, trace out the current loops for tracks that will be carrying large currents. Remember when you do this that current always travels in complete loops; it can't just suddenly stop or start at a point (on the current path illustrated above, the loop is completed via the loudspeaker). Also, identify any tracks on the circuit that connect to a high-impedance node and try to keep these as small/short as possible, preferably protected by a reference plane or track nearby.
Thanks to all for your help,
As this PCB is already produced I can't change to much. Here is what I think could help. I will cut(look the picture) main grounds(GND) from were it go around input stage and take from new GND by wires to the star ground. I removed the coil from the PCB too. Were to place the coil could be a problem. Often loudspeakers terminal are quite close and if the coils are fixded to the terminals there we have a cross talk.
For the next Layout I'll try to implement "all" (its not easy task) what was suggested here.
dado
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Old 13th September 2012, 03:46 PM   #876
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Quote:
Originally Posted by Dave Zan View Post
Hi Dado
If you feed V+ and V- in at the middle of the output transistors as Andrew T and Harry recommend then you can run the feeds to the collectors out and return the current from the emitters back on the other layer directly on top. This is what I recommended to Harry earlier - I don't see how the loop can be made any smaller than this, it is effectively only the thickness of the board.
David,

There remains a small loop area due to the finite distance between the emitter and collector terminals of the O/P devices. The good news is that you can neutralize this loop too by an 'anti loop', i.e. a loop with the same area, but which rotates in the opposite direction. Think of lemniscate or figure of 8. One enclosed area rotes CW, while the other area rotates CCW. This way the radiated fields will neutralize each other.
Of course. you also can apply the lemniscate trick at the inputs. Then they will be less susceptible to magnetic radiation (courtesy of Edward Cherry).

Quote:
As to the feedback circuit. It occurs that it would be possible to send the trace (or wire) from the centre point of the output transistors back to the input then have the feedback divider resistors there, with the earth return back to the earth point between the output transistors. This should cancel any inductive couple and not dump the feedback current into the quiet earth.

Best wishes
David
Agreed, though with only two PCB layers you can't put the Vcc and Vee traces also on each other (correct me if I'm wrong).

Cheers,
E.
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Old 13th September 2012, 04:14 PM   #877
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Quote:
Originally Posted by Dave Zan View Post
That is a return to the topic of this thread because it is my principle concern with Edmond's circuit. The hi gain is produced by the use of very hi impedance loads so I expect it will be fairly sensitive to stray capacitance and the like. I would like to use more current gain at lower impedance levels for a similar loop gain result. Not sure about how best to do that.
.............
Best *ishes
David
David,

You are right, the high impedance IPS output is of concern needs special attention: guard rings or planes. Not a serious problem, I think. Putting more current gain in the IPS is totally against the philosophy of the super TIS. If you do that, you will be confronted with other issues: an ill defined quiescent current of the VASes/TISes, which is precisely what I try to avoid with the super TIS.

BTW, did you read the recent comments on Bob's book?
I was really shocked by the ignorance of a well established audio guru regarding Iq issues with complementary VASes.

Cheers,
E.
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Old 13th September 2012, 05:28 PM   #878
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To take all that e kno here and put it in a book ith no errors, all the calculations, all the configurations, and so on, ould be an unwending task (there it is!). As I looked through Bob's book I sa it more as a compendium of somehat unorganized knoledge, intending to get much of hat e've discussed here in documentation. The book still isn't big enough to even introduce everything e have learned through netorking. As a hole e have amassed tons of collective knoledge. To do this I think e ould need something like a textbook panel consisting of Ed, Harry, and so on, and it ould need to be continually informed by forum members to ensure nothing as forgotten.

Isn't this hy e have the DIYAudio iki? Each one of us probably knos more than could be contained in Bob's book. e just need to rite it don so it doesn't disappear ith us. Collaboratively, e could surely rite many chapters in a short time.

* I should really fix that button.
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Old 13th September 2012, 05:41 PM   #879
AndrewT is offline AndrewT  Scotland
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Yes, give it priority.
Yes, stickies and articles hold info for ever, but they are so much effort to compile.

I really do thank those Members who have dedicated so much of their time to giving us those articles.
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Old 13th September 2012, 08:21 PM   #880
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Originally Posted by keantoken View Post
To take all that e kno here and put it in a book ith no errors, all the calculations, all the configurations, and so on, ould be an unwending task (there it is!). As I looked through Bob's book I sa it more as a compendium of somehat unorganized knoledge, intending to get much of hat e've discussed here in documentation. The book still isn't big enough to even introduce everything e have learned through netorking. As a hole e have amassed tons of collective knoledge. To do this I think e ould need something like a textbook panel consisting of Ed, Harry, and so on, and it ould need to be continually informed by forum members to ensure nothing as forgotten.

Isn't this hy e have the DIYAudio iki? Each one of us probably knos more than could be contained in Bob's book. e just need to rite it don so it doesn't disappear ith us. Collaboratively, e could surely rite many chapters in a short time.

* I should really fix that button.
You're right; in many ways my book is not big enough, although I do think it is well organized. I was lucky to get the 600+ pages from McGraw-Hill that I did. When I do a second edition I'll try to add more pages and add more knowledge. I'm always open to suggestions that would be good things to include there, or to better explain or elaborate on.

Cheers,
Bob
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