problems FR125SR + SI T-amp input modded - diyAudio
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Old 13th November 2007, 06:46 AM   #1
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Default problems FR125SR + SI T-amp input modded

First post and I've just just embarked on a long time dream to begin to make my own speakers.
and to start i've followed the wisdom of many DIY audio users and have begun with
a full range project.
The end plan is to make a simple portable full range speakers that can (if needed) be run from a battery.
So after reading these forums for weeks about SI t-amps i picked up one from Ebay
and purchased two CSS FR125SR's.

After i performed a change of the input caps on the T-amp i am noticing that the amp clips drastically at about 11 O clock omn the t-amp's volume pot.

Can anyone offer me advice on why this might be?
I've done a bit of searching through the forums but if it is a tired old topic please help me with where to find some advice.

I must admit that i used some cheap (A$2.00) 2.2uF caps from my local component shops becasue that is all they had. Today i bought some Aurcaps and Visaton to ensure it's not the crap caps.

What i did was to bridge c3, c4, remove R01 and R02 and to install the caps between the pot and the board.

Other than that i can see how FR DIY can be very rewarding.

hope some one can help me.

7.
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Old 13th November 2007, 02:01 PM   #2
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FR125s are inefficient speakers, so they will clip easily on the SI T amp. You need something with more power to play them louder with out them clipping.
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Old 13th November 2007, 10:25 PM   #3
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Thanks David.
I knew that they were not as efficient than the Fostex fe127's which were my other option after reading these froums for a while.
I guess they are have a 5db difference in sensitivity.
However the clipping only started when i replaced the on board input Caps.
before that, the clipping wouldn't start until ~ 2 Oclock on the pot.
which is what made me think the caps were to blame.

i might have to look around for a better suited amp that is easily battery powered.
for the FR125Sr's

Thanks
Luke.
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Old 13th November 2007, 10:37 PM   #4
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Do you have a schematic for this amp. R01 and R02 may have been a voltage divider, bringing the input level down a bit for the amp to use.
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Old 14th November 2007, 04:29 AM   #5
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The schematic of the first revision t-amp is here

and i have performed the 'stealth mod'. from the 'inputmod' section on the left of the page.

but the t-amp that i bought is another revision and according ( ) to this thread.

here

the stealth mod also places the the Caps before the RF filter. though i don't see why that should make much difference.

Thanks
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Old 14th November 2007, 06:55 AM   #6
Pano is online now Pano  United States
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I don't see how the caps would cause clipping. Except that maybe now you have more bass getting to the amp that wasn't making it thru before. But I've not had that trouble on a flat signal.

Do you have a bass boost or loudness control somewhere before the amp?

Generally these amps don't clip badly until about 2 o'clock, as you mentioned. Even if you are clipping a peak or two here and there, you likely won't notice.

Might be that you did something odd with the input circuit. Photos are in order, here.

If a mod would be kind enough to move this over to the class-d forum, maybe the group mind there can figure it out.
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Old 14th November 2007, 05:55 PM   #7
chrisb is offline chrisb  Canada
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Quote:
Originally posted by davidallancole
FR125s are inefficient speakers, so they will clip easily on the SI T amp. You need something with more power to play them louder with out them clipping.

The lower sensitivity of the CSS drivers is a price paid for the bass extension and lower distortion - unfortunately in the case of the FR & WR125, the XBL motor's strength appears to surpass suspension's excursion capacity.

What you're hearing may very well not be solely the amp clipping, but the suspension's distress. This was very clearly noted on a pair of bipole MLTLs that "mathed -out" well enough, but perhaps didn't adequately load the driver around or below its Fs.

Let's say a consensus description of the noise was of an anatomical nature.

It's been my experience this is not an issue when the WR/FR are in a sealed or adequately damped enclosure (such as the mini-onken) or with the Extremis.
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Old 14th November 2007, 09:07 PM   #8
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QUOTE]Originally posted by chrisb


What you're hearing may very well not be solely the amp clipping, but the suspension's distress. This was very clearly noted on a pair of bipole MLTLs that "mathed -out" well enough, but perhaps didn't adequately load the driver around or below its Fs.

It's been my experience this is not an issue when the WR/FR are in a sealed or adequately damped enclosure (such as the mini-onken) or with the Extremis.
[/QUOTE]


Interesting. I think this may be as you say at least partially the case. I currently have the FR125sr's in temporary enclosures of PVC piping. i have stuffed very full of medium density foam.
I'll seal them tonight and test again.

Panomaniac:
I've got some Auricaps coming today, so i'll open it up and take some photo's too, though i'll have to admit now that my soldering is a shocka!.

Thanks again all.

Luke.
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Old 14th November 2007, 09:22 PM   #9
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I am not sure exactly what you have done. Did you say you removed R01 and R02. Do you have a schematic for your revision or board. Those kind of look like the feed back resistors or something. Can you draw a schematic showing what you have done for the input to the amp with your capacitor and resistor mods?
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Old 14th November 2007, 11:53 PM   #10
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Quote:
Originally posted by davidallancole
I am not sure exactly what you have done. Did you say you removed R01 and R02. Do you have a schematic for your revision or board. Those kind of look like the feed back resistors or something. Can you draw a schematic showing what you have done for the input to the amp with your capacitor and resistor mods?
Hi David
Since i have the second revision of the t-amp board. i removed R01 and R3 and bridged C4 and C3.
the new layout of the t-amp R2 board was shown here:new cap layout. so i removed and bridged accordingly. i hope.......

I then placed the connected the 2.2uF caps in between the board and the volume pot for each channel. according to this diagram:stealth mod

i connected them inline with the ribbon cable temporarily, but today received my Auricaps, so i will connect then directly to the board and a new higher quality pot if i have not botched the job.

Still no photo's since i'm at work, but i'll try and shot tonight.

Thanks

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