Just fired up my new dual mono lm1875 Gainclone. - diyAudio
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Old 12th January 2004, 06:11 PM   #1
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Default Just fired up my new dual mono lm1875 Gainclone.

I have just completed and fired up my latest Gainclone.
I was using the Rod Elliot LM1875 project before with a single power supply and nothing special in the components. As a matter of interest I just replaced a EI transformer with a Torodial and was amazed at the difference it made- for the better.
Anyway my new one is none inverting and uses P2P. I have used dual torodials and ultra fast soft recovery diodes. There is a cap at the input ,4.7uf at 160V non polarised, but no series resistor. I have used a carbon film to reference the input, but other than that I have used metal film 1% resistors. The zobal uses a 0.22uf wima cap.
First impressions are that it is a great little amp. Much softer sounding than the previous Gainclone- but with much more detail and depth.
The great thing was that it fired up first time with no problems. I plugged in my crap test speakers and got no cone movement, so went straight to my good speaker without even measuring DC.
The only problem I have sofar is a noticable hum, but I have a feeling that this is down to the proximity of the amps to the torodials (only inchs). One channel is closer and suffers more from the hum. I will try a copper screen.
I am really suprised at how much of an improvement I have got over my orginal circuit board Gainclone.

Shoog
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Old 13th January 2004, 05:10 PM   #2
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Would you believe that its actually wisper quiet. Turns out that the hum was all in my BOZ preamp, and the extra gain just made it worse. After a bit of tinkering I tracked the problem down to the wiper earth on my volume control which was passing to near to my transformer. Tying it back eliminated that problem.

This is the best result I have ever had.
I took my time building this one, and the patients payed off.

Ready for my tube buffered gainclone now.

Shoog
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Old 13th January 2004, 06:02 PM   #3
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Quote:
the patients payed off
They always do when the doctor is good




Quote:
I am really suprised at how much of an improvement I have got over my orginal circuit board Gainclone.
Are you trying to get the manufacturers of unnecessary PCBs out of business?

Well done!
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Old 13th January 2004, 06:04 PM   #4
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I think the board was not the only difference if I really READ the post.

The "unnecessary" PCB's might have had the least influence on the whole. Shooq made an extirely different amp certainly concerning the power supply.

Out of context might have been a better answer.
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Old 14th January 2004, 01:31 PM   #5
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I have no beef with the PCB as it got me going again after a number of failed P2P attempts. However after trying the concept with the PCB I went to a lot more trouble over all the parts. Certainly I would guess that the power supply would be making the most improvement, but my latest version is a lot more stripped down than the PCB version, with no resistors in the direct signal path.

I would say that for someone wanting to build their Gainclone "NOW" a PCB is much more likely to be a success. It then gives you enough breathing space to do the serious amount of reading needed to get the P2P working to it best potential.

Thats my thought anyway.

Shoog
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Old 19th January 2004, 06:09 PM   #6
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I was finding my new amp just a little harsh, so after reading through a few threads about LPF I decided to try out the concept.

now that the amp is assembled it is nearly impossable to do a proper LPF implementation because it would require working in to confined a space. As a compomise I decided to solder a 300pf cap across the 20k resistor to ground on the input. I know I should have preceded it with a series resistor on the input, but as I said it was physically nearly impossable. To my ears it seems to have removed all trace of harshness, without the overall clarity suffering. I need to do some serious listening to check out my initial impressions. I would say that it seems to have removed some definate noise component from high frequencies.
The thing is I know this is not an ideal arrangement as I am relying on the ouput impedence of my Bride of Zen preamp (about 700ohm) to set the cutoff point, but with so low a cap value I am assuming that the roll off is so high that a certain amount of drift in this value wouldn't be that critical.

Anyone care to make an obervation (I don't want to reopen the whole LPF argument again - I simply want to know what effect my modification might be having in my amp).

Shoog
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