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Old 29th January 2005, 05:50 AM   #11
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Hi Steve,

No, I didn't move anything around. I did measure the the resistor and it reads the same as all the rest. It's very weird. Nothing gets hot or anything, as a matter of fact, the heatsinks barely get warm. I fed the same signal to all three channels and used banana plugs on a speaker and switched back and forth between the three channels and couldn't hear any difference. If I had a scope maybe something would show up but I can't hear it.

For now, I'm satisfied.

Thanks for the help,
Terry
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Old 30th January 2005, 10:02 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
I am interested in your comment re bass cf Soundcraftsman. Could the soundcraftsman have a bit of bass lift or has it got a higher input impedance? I'm looking for excuses because I want to believe that the dual LTP has good bass but I fear others have reported a similar view to yours.
Secondly you report 8 * 15mF in your amp. Is this total for all three channels or per channel or per rail or (now getting silly) per rail and per channel?
I have a little 110w into 8R amp that has 21mF per rail per channel and that is a big bass improvement over 10mF.
Yours has a much higher current capacity.
regards Andrew T.
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Old 30th January 2005, 03:24 PM   #13
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Quote:
Originally posted by still4given
If I had a scope
Terry,

you can download small scope programs for free that allows you to use your pc and microphone entrance of the soundcard as a simple single channel oscilloscope.

If you have a way of generating a sinoidal signal for the entrance of your amplifier, with help of your computer and such a program you are able to check the output signal.

jacco
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Old 30th January 2005, 05:36 PM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
further to bass of a Leach /clone amp; what RC constant have you used for the VAS stage hum filter? (Leach suggests 50mS = 20Hz).
Could this have an effect on bass?
regards Andrew T.
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Old 30th January 2005, 06:23 PM   #15
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Quote:
Originally posted by AndrewT
Hi,
Can't solve this problem but offer some advice to help next time.
ESP does not fit protection on the VAS. It is only one to92 transistor and 2 resistors. You could retro fit it, either hard wired or on a tiny piece of stripboard. Each time you overload the output you risk losing VAS and outputs again.
regards Andrew T.
I'm not sure what you mean by protection on the VAS, do you mean output current limit implemented by gate voltage limiting? This is implemented at least in a short term fashion by the gate-source diodes on the output devices.

Even without this protection, if you short the output and manage to blow the output devices the VAS will get away unscathed. This is because MOSFETs -- unlike BJTs -- the gate is isolated from the rest of the device.
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Old 30th January 2005, 06:26 PM   #16
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Quote:
Originally posted by AndrewT
Hi,
I am interested in your comment re bass cf Soundcraftsman. Could the soundcraftsman have a bit of bass lift or has it got a higher input impedance? I'm looking for excuses because I want to believe that the dual LTP has good bass but I fear others have reported a similar view to yours.
Does the Soundcraftsmen have dual LTP? P101 doesn't.
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Old 31st January 2005, 05:38 PM   #17
AndrewT is offline AndrewT  Scotland
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Hi all
re post 12 & 14;ignore my bass response request. I got my threads mixed up SORRY.
VAS protection is a 2 transistor ccs wrapped around the VAS transistor. One extra transistor and an emitter resistor.
I think VI limiting applied to BJT is exactly equivalent when applied to FET output, because it is driver drive limiting that is applied.
When VI limiting is applied across the drivers to limit current in the output it effectively shorts the driver base connection to the output connection. All the current feeding through here comes from the VAS. The VAS will be running hi current and hi voltage.
But the VAS current varies either side of the preset bias and in extremes could range fron zero current to two times bias under normal operating condition. It follows that the VAS protection should limit at 2.2 to 3 times bias current to allow normal driving into low loads. This is easily achieved by splitting the emitter resistor say 40% and 60% and detecting volts drop on the 40% end nearest the Vrail. The current sink using ccs automatically limits itself to bias current combined with hi voltage and should be designed to survice a short circuit with hi input signal.
If all that is unclear look up Leach (low tim), he shows the circuit modification and describes the operation. It is commomly adopted in many designs (I have it in all my Crimson).
regards Andrew T.
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Old 31st January 2005, 06:36 PM   #18
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I see now. Another reason to use a CCS in the VAS rather than a bootstrap, then.
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