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Leach Amp Group Buy

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#3
Mr. JLeaman,
I read the thread and check Jens' web site. I am somewhat unsure. Which output devices are used in this amp (power output too)? Also the heatsink requirement for convection cooling?

Second, do you have an idea what the parts will cost?
I might be in for 2 or more if we also have a GP for parts.
 
#10
Hi Jens,
2 suggestions
1. add a third input to bypass the DC blocking caps (for a sub).
2. Is there room to squeeze in the traces to provide the dc setting as done in KSA50 (4 resistors, 2 diodes & 1multiturn pot)?

I am having trouble reading the PCB pdfs. The black background obliterates the traces except at 300% and above. Can you do a colour for the background (backplane)?
 
#11
Andrew,

1) ok, I will use a 3 pin input header, and use on as DC coubled input.

2) There is no need for a DC pot, I have not measured above 40mV DC on the output on any of the 8 channels I have build using 1% resistors and unmatched semiconductors.

About the PDF, I can make a better resulution (BIG files 🙁), or I can post GIF pictures in stead of the PDFs

\Jens
 
#12
DC setting

Hi AndrewT.
I build the first Leach in '81 and one amp had a little offset, 73 mv.
To make this zero i added an (outboard)offsetcircuit like the KSA50.
Before building the second channell i paired the small signal transistors and when it was finished the offset was about 15 mv.
Perhaps Jens can give us his amps offset voltage?

Good luck, Loek

Jens is very fast, he answered before I could post....
 
#15
dc offset

Hi Jens, I dont 'think it is needed. But you can indeed use this circuit.
In the Krell it is connected to the feedbackside of the input circuit.
I made it more simple without R104 and R106 and placed it to the input side. R131 is 47k in my amp and I also placed a small cap from the wiper to ground.
Loek
 
#16
1 The added pads on the boards for raised front end voltage are very nice.

2 Would it be an idea, (and possible), to add Michael's triple soa protection to the board ?

3 with 10 components added for DC-zeroing you might as well make it an active offet regulation with an OP** opamp.
With the current layout i dont see the room for adding that many components. But i am not a pcb-designer.
 
#18
pooge said:
Have you sought any comment from Prof. Leach re the boards and the layout?
Click to expand...

On the Leach thread i suggested to ask BrianGt to approach Professor Leach for advice on the design.
Brian is a former student of Marshall Leach, and still resides in Georgia afaik.
Personally, i would be more than interested in what Professor Leach would suggest for the feedback values when using higher bandwidth devices. imo, this is/could be the V5 design.
 
#19
jacco vermeulen said:
1 The added pads on the boards for raised front end voltage are very nice.

2 Would it be an idea, (and possible), to add Michael's triple soa protection to the board ?

3 with 10 components added for DC-zeroing you might as well make it an active offet regulation with an OP** opamp.
With the current layout i dont see the room for adding that many components. But i am not a pcb-designer.
Click to expand...

1) Well, you always wanted those anyways, so I figured what the heck.

2) I already made a dual slope soa protection awalible on the board by adding R68 and R69

3) I played arround with the DC circuit last night, but could not fit it without redoing the input section. Should I bother or just leave it the way it is?

\Jens
 
#20
JensRasmussen said:
I played arround with the DC circuit last night, but could not fit it without redoing the input section. Should I bother or just leave it the way it is?
Click to expand...

speaking for myself:
i will be matching devices anyway, if needed i'll measure a number of resistors and pick the one that makes DC offset minimal, or pick a device that does that .
If the outcome doesnt please me i'll add an active offset regulation, with an amplifier as the Leach i have no problems with that.

The front part i like the way it is, is is closely nested, nice symmetrical, well separated from the current part by the big capacitor spacing.
Leave it !
 
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