value of capacitors in signal path - Page 3

Mooly · 11th February 2010, 07:05 AM

You need to see if there is any DC voltage on the output of the second OpAmps. If yes and it's more than a say 5 mv, the caps should stay.
The "first" 100uf... measure DC volts on it from the DAC.

Changing the 6K8 to 4K7 alters the AC response of that stage due to C352 and C353. You have increased the bandwidth... letting more HF "hash" through.

Oystein · 11th February 2010, 07:24 AM

I can understand I need some dc protection but only 1 set of capacitors should do fine ...
If I keep the set after the amplification it doen not matter that much if the is a little (amplified) dc voltage in front of it.
The first stage has little amplification so any present dc from the dac will be small.

Mooly · 11th February 2010, 07:33 AM

The DAC output "may" be at several volts by design... so measure it on the first 100uf cap to be absolutely sure.
The offset voltage of the opamps is an issue as well. With the caps in place there should ideally be zero DC volts at the output of each opamp. FET devices will give this, bjt's won't .

Mooly · 11th February 2010, 07:36 AM

In fact looking more closely at your circuit pin 13 of the DAC is shown as being at 2.5 volts... but measure it for yourself

Oystein · 11th February 2010, 07:43 AM

You are right ... the center voltage on the analog out is 0.5 * Vcc
I will measure the voltages before I will make any changes.

11th February 2010, 07:05 AM	#21
Mooly diyAudio Member Join Date: Sep 2007	You need to see if there is any DC voltage on the output of the second OpAmps. If yes and it's more than a say 5 mv, the caps should stay. The "first" 100uf... measure DC volts on it from the DAC. Changing the 6K8 to 4K7 alters the AC response of that stage due to C352 and C353. You have increased the bandwidth... letting more HF "hash" through. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

11th February 2010, 07:33 AM	#23
Mooly diyAudio Member Join Date: Sep 2007	The DAC output "may" be at several volts by design... so measure it on the first 100uf cap to be absolutely sure. The offset voltage of the opamps is an issue as well. With the caps in place there should ideally be zero DC volts at the output of each opamp. FET devices will give this, bjt's won't . __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

11th February 2010, 07:36 AM	#24
Mooly diyAudio Member Join Date: Sep 2007	In fact looking more closely at your circuit pin 13 of the DAC is shown as being at 2.5 volts... but measure it for yourself __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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11th February 2010, 07:24 AM	#22
Oystein diyAudio Member Join Date: Jun 2009	I can understand I need some dc protection but only 1 set of capacitors should do fine ... If I keep the set after the amplification it doen not matter that much if the is a little (amplified) dc voltage in front of it. The first stage has little amplification so any present dc from the dac will be small.

11th February 2010, 07:43 AM	#25
Oystein diyAudio Member Join Date: Jun 2009	You are right ... the center voltage on the analog out is 0.5 * Vcc I will measure the voltages before I will make any changes.

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