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Help modifiying schematic

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#41
crispycircuit said:
I'm in the process of finding decent chassis for my mono's. I like some room to work with. This project is going to be more expensive that planned but really fits my needs... Your help is wonderful......
Click to expand...

Why dont you build your own chassis? I'ts a good experience 😀

I wanted to build it with bronze... but it's really expensive.

Dont forget (like I did) to put the PSU tranny and the output tranny 90° rotated!
 
#43
zafira1981 said:
I've made some modifications to the schematic. Here are the changes:

1- Added grid stopper on EF86
2- Added grid resistor to 1st grid of EF86
3- new CCS with BD139
4- New B+ voltage

http://img116.imageshack.us/img116/8533/ampqu3.gif

cheers
Click to expand...

I like it. It looks like one of the best PP schematics I ever saw.

However, voltages on transistors you've measured look strange...
 
#46
Wavebourn said:


Voltage on emitter of the upper transistor is higher than on the base. I mean your hand-written sketch on the previous page.
Click to expand...

You're right!! the VBE of that transistors seems to be 8.6V :xeye:

I'm trying to figure out what's wrong. The IC of both transistors are 6mA, I'm sure of that. I suppose that tr2 is working "forced" by TR1 but that VBE still look bad.

I tested both transistors and they are fine. I will read some theory about CCS and post again.

thanks!!


EDIT: I noticed that bd139 has a small VCEmax (60V), I have 2 TIP122 wich VCEmax is 100V. They're darlintongs, not the best but that's what I have now at home.

I will try another CCS.
 
#50
It's working!!!! 😀

With all this modifications I had a innnnnnnnnteresting improve of bass response. I've built only one channel.


The CCS appears to be doing well, the same for the bias-circuit of the KT's.

B+ is 494V when amp is running.

KTs are reaaally hot :hot: but anode current w/o AC input are OK (50mA each)

I will take some pics to post here 🙂
 
#52
besides the bad-design of not rotating 90° the PSU trans and the output trans, there is no hum. I still have to use twisted cables for the heaters and re-order the components (I'm building it only as a prototype).

There is a liiitle hiss. Maybe because I'm using large cables and the CCS is mounted on a protoboard. But it's audible only if you stand really close to the tweeter.
 
#56
new update


I've installed trueRTA program to make a frequency response graph. I cant make it work, when I connect the loop to calibrate, I't seems that the sound card become unstable and oscillate. I've searched to resolve this problem but I couldn't fix it. I'm still trying.

About my small-current choke, I was wandering if I can lower the inductance changing the winding for a new one, with ticker cable and less turns.

The actual is 10H 250mA. Maybe take it to.. 4H 1A? I have to check my old college notes about building transformers.

my PSU is 220uF - 10H - 220uF

what do you think?
 
#57
zafira1981 said:
I've installed trueRTA program to make a frequency response graph. I cant make it work, when I connect the loop to calibrate, I't seems that the sound card become unstable and oscillate. I've searched to resolve this problem but I couldn't fix it. I'm still trying.
Click to expand...

Please check that you have disabled (mute) the line and mic sources from the playback mixer. Othervize you will have an internal loop (=high pitched whine).

SveinB.
 
#58
Svein_B said:


Please check that you have disabled (mute) the line and mic sources from the playback mixer. Othervize you will have an internal loop (=high pitched whine).

SveinB.
Click to expand...

that was the first thing I did.

If I mute the line-in in the playback, I do not know why, it mutes the recording too (not in the checkbox but I cant record if the playback of line-in is muted). Maybe I will change the sound drivers.
 
#59
well, at last, I did some measurements. I dont know if they're OK, but they make sense.

new feedback: 20 log (4700/100) = 53dB
Po = 20W

Transfer
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THD
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Noise
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Intermodulation
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For the first test, the amp seems to be working very well!!
 
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