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Old 9th December 2004, 03:22 AM   #11
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(from the peanut gallery)

My limited experience is that the LM3875 just doesn't love 4ohm loads like 8ohm. The bass is where it fails first. I then built the BGT4780 and it was a different story all together, but I was thinking it was just tons more power and that was the difference. This newer kit however, has the Zobel network.

Are you guys saying that the Zobel is at least worth a try and perhaps this "voicing of the amp" is a possible next step for the 3875 to be happier with a 4 ohm load? Are the values the same ie, 2.7ohm R and .1uF cap.... and if I might ask, what other values are logical to try?

(wish I knew what I was talking about)
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Old 9th December 2004, 05:50 AM   #12
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Thanks for all of the quick answers.

I will give it a try this weekend when I get a little bit more free time, I hope.

Thanks for the info on the BGT4780 kit. I would like to give one of those a try once he gets them back in stock. I would move the 3785 to my computer then.

Craig
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Old 9th December 2004, 07:29 AM   #13
XELB is offline XELB  Portugal
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Quote:
The bass however is a complete mystery to me. What is there is very tight and controlled sounding, but it is lacking any sense of impact or force. This seems to hold true at whatever volume I listen at. My speakers are very capable of delivering good tight bass, but it is not there now.

Witch capacitors are you using ?

In my opinion and becouse you are using 4 oms speakers maybe you need capacitors with more capacity.


I have a 6 oms speakers and they Rock with a amp of 300VA and 40000nF of capacity.


Regards.
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Old 9th December 2004, 08:37 PM   #14
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I am using the caps that come with the BrianGT kit.

They are "1500 uF 50v Panasonic FC Capacitors"

I know that the boards are set up to accept some different types of caps. I would appreciate any specific suggestions that anyone would have on a different size or type of cap.

Craig
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Old 9th December 2004, 09:07 PM   #15
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You know what guys? The zobel network actually made things worse for me. With my lm3886, the highs became a little softer (that was to be expected), but the worst thing is that the soundstage collapsed. It's like the amp became mono..


Seriously, only use the zobel if you're having problems with the amp.. otherwise best to leave it out, IMHO. (at least, from my experience)

Take care,
300_baud
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Old 9th December 2004, 11:12 PM   #16
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Yeah tried a zobel again, but this time i used:
1) 0.47 uf Wima Cap
2) 2x Holco 10 ohm 1/4 watt in parallel (5 ohms as a result)

And the soundstage was okay.. i think the reason why the zobel didn't work for me before was because i was using a generic 1 ohm carbon resistor...

With these settings, the treble has a little more perk. I'd have to listen more for bass.
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Old 15th December 2004, 07:09 PM   #17
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I added the zobel and finished the case this weekend.

One word on the zobel WOW!!! The bass is back.

The only other change that I see in the amp with the addition of the zobel is that the chips now run a little bit hotter. About 138 deg f with the zobel added, before about 120 deg f.

Thanks for everyones input and advice once again.

Here is a quick picture of my finished chasis.
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Old 15th December 2004, 07:51 PM   #18
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I didn't like the effects of a Zobel, but I only tried 1ohm and 0.22uf. Seemed to kill something in the overall presentation and made things sound harsh.
I respect other peoples positive comments on different values, and someday will try them.

I use a 10ohm resistor with 10turns of enamel wire around it, to little ill effect. My speakers are easy to drive so I can't comment on if this works on hard to drive speakers.

Shoog
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Old 16th June 2014, 08:12 PM   #19
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Default Zobel network

Quote:
Originally Posted by HawkeyeStoob View Post
I have listened for about 60 hours now and my initial listening impressions to the amp have given me somewhat confusing results.

From the first note the midrange has been outstanding. Open, full, and very detailed. The high end started out very thin. Cymbals and similar instruments were muted and did not carry the tones well at all. At about 20 hours that really started to come around. The notes seemed to open up and sustain much better giving the music a natural and lifelike quality. I am confident that this will continue to keep improving with a little more time. The bass however is a complete mystery to me. What is there is very tight and controlled sounding, but it is lacking any sense of impact or force. This seems to hold true at whatever volume I listen at. My speakers are very capable of delivering good tight bass, but it is not there now.

This leads me to my questions. I have read several posts regarding the use of a zobel network and its advantages or disadvantages. A couple of the posts seem to indicate it has a positive effect on bass output of the gainclone amps.

I am curious to hear what others have found to be the results of this addition.

I would also like to hear from anyone who has installed it and then removed it as to what if any effect it made on the sound of the amp.

I am using a LM3875 kit from Brian with an Avel 250VA 18V+18V transformer driving 4 ohm speakers.

Thanks for your thoughts,
Craig
A zobel network will only work properly if you are using the right component values. The component values are different for the 4 ohm speakers compared to the 8 ohm speakers. A zobel's job is to compensate for the rising impedence of the the speaker usually the woofer at higher frequencies. Compensating would help provide a stable load for the amplifier. This would be true for an active setup. For a passive setup i.e. if you have a passive crossover then it would probably make more sense to place the zobel after the passive crossover as you are trying to correct the impedence of the speaker and not the crossover. You zobel's aim should be to keep the load impedence at 4 ohms at all frequencies BTW the LM3875 wasn't really designed to drive 4 ohm speakers it is more suitable for 8 ohm speakers and likes to run on higher voltages somewhere around +/-35V. At lower voltages the bias currents will be affected and the sound quality will deteriorate. For 4 ohm speakers you are better using the LM3886 which is designed to drive 4 ohm loads and has higher bias currents which are more suitable for lower voltages e.g. +/-26V.

Last edited by AudioEngineer; 16th June 2014 at 08:16 PM.
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Old 17th June 2014, 09:45 AM   #20
basi is offline basi  Croatia
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Quote:
Originally Posted by HawkeyeStoob View Post
I am using the caps that come with the BrianGT kit.

They are "1500 uF 50v Panasonic FC Capacitors"

I know that the boards are set up to accept some different types of caps. I would appreciate any specific suggestions that anyone would have on a different size or type of cap.

Craig

Hi!!!
If you need more bass and better control of sound then add 10 000uf to your
power supply.
And increse input and feedback cap.
Example 10uf input,100 uf feedback.
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