Reducing the gain - LM3875 and alternative diodes :)

[quickshift]

Hello

I have searched but can't find the answer I'm afraid.

I want to reduce the gain on a new LM3875 amp I'm planning to build (I have a spare set of Rev 3 chipamp.com boards I want to experiment with )- I think the gain is R2 / R3 which = 32.45 ATM - Is that right?

If that's correct, is it better to get a lower value of R2 (currently 22.1K) or a larger value of R3 (currently 681R)

Also....

Is it OK to use these diodes instead of MUR860's? - they are a bit cheaper and faster. I don't really have a clue about the spec except for the number of pins

Manufacturer: STMICROELECTRONICS

Manufacturer Part Number: BYW80-200

Description: DIODE, FAST RECOVERY 8A; RoHS Compliant:YES; Voltage, Vrrm:200V; Current, If av:8A; Current, Ifs max:100A; Time, trr max:35ns; Voltage, forward at If:1.05V; Case style:TO-220AC; Diode type:Fast Recovery; Pins, number of:2

[Leolabs]

why not try them and listen for the differences???

[wintermute]

Not sure about the gain, the ratio between the resistors is what sets it, but I don't know what the difference it makes whether you increase the low value or decrase the high one..... I noticed GregE in another thread was using 320K /10K Greg any comments???

for the diodes, I checked the datasheet and it didn't mention soft recovery. from what was mentioned in a thread over on electronics and parts, it is in fact soft recovery that is more important than fast recovery..... you might want to have a read of this thread ---> Ultrafast rectifiers ?

Tony.

[quickshift]

Thank you.

I'll try them if the diodes could work, I didn’t want to if they definitely wouldn’t. The summary details at the place selling the diodes say that they are soft recovery. I'll try to get contact details for the manufacturer and check with them first if it doesn’t confirm that on the datasheet. Many thanks for the info.

I found a thread about diodes in power supplies, there were loads of diagrams I didn’t really understand but the link you have provided is very useful – cheers.

I could experiment with resistor values as they are really cheap and I can work out the ratio easily.

[Christer]

http://www.st.com/stonline/products/...2967/byw80.pdf

There is no info in the datasheet to confirm that they are soft recovery. They are fast recovery though. Maybe it can differ between brands if they are soft or not, but ST are not according to the datasheet.

[quickshift]

Hmm, OK. Soft recovery is the important bit I think so I'll definitely have to check - I guess it would say on the sheet if they were.

Thank you.

[wintermute]

I don't think there is a problem of the diodes not working. Both the current and voltage look high enough. You just won't get the advantage that soft recovery offers.

I'm not sure where you are buying them from but Farnel UK's price on them is 1.11 pounds, and the MUR860's are 98 pence.....

you might want to try these ----> http://uk.farnell.com/jsp/endecaSear...U=250697&N=401 as recommended in the thread I posted (I'm actually using the BYV32E-200's but these won't fit the Brian GT PS board because they have two diodes per package and 3 pins).

the other advantage of the phillips ones I just posted the link for is that the price is only 59 pence each (36 pence for quantities of over 25).

Tony.

[quickshift]

That's odd...

The 860's were £1.70 ish last time I looked on the Farnell site...

The phillips ones look good, I'll try them, many thanks again

I want to do a few experiments and build something "my own" based on a kit to try to learn more so I'll be trying different components. I'm sure the kit parts are well tried and tested and all that but I like customising things and making them a bit different - provided they still work...

[Greg Erskine]

Originally posted by wintermute
noticed GregE in another thread was using 320K /10K Greg any comments???

When I build the amp in question, these were typical values for inverted gainclones. Things have changed dramatically over the years. I have tried other values but when it comes to chipamps I seem to be a little deaf.

Originally posted by wintermute
I... as recommended in the thread I posted (I'm actually using the BYV32E-200's but these won't fit the Brian GT PS board because they have two diodes per package and 3 pins).

You could cut off one of the outer legs and squeeze them in if you have to. When I compared MUR860s with a generic bridge I couldn't hear a significant difference, but others can.

As far as gain is concerned because of the vast differences people have reporting using with good results, it seems you try any 2 resistors (within reason) as long as the gain is above the minimum for the chip in question.

regards

[quickshift]

Originally posted by Greg Erskine


As far as gain is concerned because of the vast differences people have reporting using with good results, it seems you try any 2 resistors (within reason) as long as the gain is above the minimum for the chip in question.

regards

Excellent, thank you.

I'll get a range of resistors in and experiment.