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Old 29th July 2010, 07:23 PM   #1
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Question Noobie needs help on ebay amplifier

Hello!

A few months ago I bought this kit from ebay: POOGE-2 Mosfet Power Amplifier.
When I first assembled it I didn't like the sound - highs too harsh and agressive.
I then replaced the Toshiba mosfets for ON Semi Bipolars (MJL21193/4) and changed R34-37 for 0,1 Ohm and R30-33 for 10 Ohms. The sound is much better now but is still a little harsh. I don't know if the values I choosed for the resistors are good.
I can't find any reference for this circuit on the internet and because I'm very noobie I don't know if the values of all the other parts are good or maybe there's a mistake I can't see... or room for improvement.
There's also a jumper - what is it for? Should I leave it opened or close it?

Thank you in advance for your help!

Regards to all,
Paulo.
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Old 29th July 2010, 07:34 PM   #2
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Default Here is the BOM:

I forgot the BOM, here it goes:
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File Type: pdf Esquema.pdf (97.8 KB, 68 views)
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Old 29th July 2010, 08:09 PM   #3
LAJ is offline LAJ  United States
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I always thought FET amps sounded better than bipolar ones. No doubt your bias adjustment will be different with bipolar outputs. Did you check your bias?
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Old 29th July 2010, 08:15 PM   #4
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Yes, I rise the bias up to 300mA and the bipolars sound much better with only 100mA bias. Maybe the mosfets are fake? They really sound to much bright and harsh (odd harmonics, distortion perhaps?)

Regards,
Paulo.
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Old 29th July 2010, 08:23 PM   #5
LAJ is offline LAJ  United States
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I couldn't find any information on a pooge amplifier. I would get a test oscillator and look at it on a scope for distortion. My guess is the outputs that it came with are not fake but your bipolar ones could be. How about loading? What kind of load are you using?
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Old 29th July 2010, 08:37 PM   #6
LAJ is offline LAJ  United States
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Default Found the Pooge-2

Looks like a good amp. But I would stay with the topology that it came with. I always thought FETs were superior to bipolars.
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Old 29th July 2010, 09:13 PM   #7
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I got the bipolars as samples from ON Semi, they can't be fake.

Quote:
. I would get a test oscillator and look at it on a scope for distortion.
I don't have an osciloscope...

Quote:
I always thought FETs were superior to bipolars.
The mosfets the kit came with are types for audio... it should't sound that harsh... treeble feels piercing the ears... so I think or they're fake or there's some error on the schematics, maybe values are wrong... maybe the decoupling 100uF capacitors should be a higher value... I don't have yet the knowledge of electronics to know if the parts values are good... Maybe the vishay-dale RN60D resistors are fake? I'm trying to figure out what the problem might be...

Quote:
What kind of load are you using?
I'm using Kef Cresta 10 8 Ohm speakers.

Regards,
Paulo.
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Old 29th July 2010, 09:25 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

FWIW optimally biasing bipolars is far easier than FETs, they are worse.
FWIW bipolars are far better devices for budget amplifiers than FETs.

/Sreten.

edit : and FWIW driver stages due to current requirements are not
compatible, though at higher frequencies they are somewhat.

Last edited by sreten; 29th July 2010 at 09:43 PM.
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Old 29th July 2010, 09:32 PM   #9
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One must remember that a amp is only part of a system, isnt it perhaps your preamp or cd player that sound harsh. What amp did you use before this one ?? Perhaps this is what this amp sounds like and the rest of your setup just doesnt match well with it.

I agree that mosfets do sound more brighter in mids and highs than bipolars but then again they are weaker sounding in the bass region, pick your poison, but this is not the problem if the harshness stays there. Perhaps the amp has slight oscilation, youll have to put it on a scope. If all is ok on scope the only thing you could do is to try some modifications, especially with the compensation.
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Old 29th July 2010, 09:39 PM   #10
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I'm using my laptop as source, the DVD player, and the mp3 player. They all sound harsh with the Pooge. My previous amps were the Nad C320BEE and the SKA GB150D. They both sound smooth and a lot nicer in comparation with the Pooge.

I supose I can't connect the amp to the pc sound card (with a voltage divider) and use the setup as a scope?

Thanks.
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