Matching a commercial preamp - adcom

schmalex · 20th June 2005, 06:38 PM

Hi, does anyone have any experience matching a chip amp to an adcom preamp? I plan to build my first amp in the next month and want to buy a preamp to go with it. I don't have a lot of money, so a gtp-400 or 500 seems like a good choice. Of course, I know I should build my own preamp, but the convenience rules on this one.
Thanks,
Alex

Nuuk · 20th June 2005, 09:22 PM

It's mainly a question of matching the gain of your chip amp with the gain of the preamp so that you don't have too much. Too much gain would give you little useable range on your volume control.

Can you tell us what is the gain in your chosen preamp?

schmalex · 21st June 2005, 08:36 AM

Well, that's a good question. The manual for the GTP-400 doesn't list gain, but it does list Loudness (volume at 9'oclock) - 100Hz.............+5dB
20Hz.............+10dB

I assume that's the same as gain, yes?

Nuuk · 21st June 2005, 09:51 AM

No, they are talking about a boost (optional) of the lower (bass) frequencies when listening at lower volumes.

It's not absolutely essential but if you want to do things properly, you will need to know how much the input signal to the pre-amp is increased (gain) before the output to the power amp. I'm not so sure I would trust a hi-fi component supplier that couldn't supply this basic information!

I know that you say 'convenience' is an issue here but why not just build your Gainclone, use it and when it is convenient, build a pre-amp? If you build the pre-amp shown on Decibel Dungeon, it has a unity gain so you wouldn't need to change the gain in the power amp section! But if you wanted gain in the pre-amp, you could easily change the gain of the power amp to suit. Having said that, you could probably just buy that pre-amp and tailor the gain of the GC accordingly.

schmalex · 21st June 2005, 05:19 PM

I'm trying to attach the specs page from the user manual.

How do you change the gain in chip amp? I'm planning to build BrianGT's LM3875kit.

Thanks for the answers Nuuk. Your website's been very helpful.

Nuuk · 21st June 2005, 06:18 PM

I can't tell from that spec what the gain is!

You can alter the gain on the GC by changing the feedback resistor or input resistor, providing that you don't lower the gain below 10 and keep a high enough input impedance.

If your heart is set on the pre-amp, put the two together as they come and see how they are together. If you find that it is hard to alter the volume precisely, lower the gain on the GC as described above.

analog_sa · 21st June 2005, 08:08 PM

Quote:

I can't tell from that spec what the gain is!

Why?

It appears to be x6.45

carlosfm · 21st June 2005, 09:08 PM

Quote:

Originally posted by analog_sa
It appears to be x6.45

x6.45161290322580...

High gain...

Nuuk · 21st June 2005, 09:10 PM

OK we want no smart-***** here chaps. Tell us how please!

carlosfm · 21st June 2005, 09:15 PM

Quote:

Originally posted by Nuuk
OK we want no smart-***** here chaps. Tell us how please!

2V / 310mv.

20th June 2005, 06:38 PM	#1
schmalex diyAudio Member Join Date: May 2005 Location: Seattle, WA, USa	Matching a commercial preamp - adcom Hi, does anyone have any experience matching a chip amp to an adcom preamp? I plan to build my first amp in the next month and want to buy a preamp to go with it. I don't have a lot of money, so a gtp-400 or 500 seems like a good choice. Of course, I know I should build my own preamp, but the convenience rules on this one. Thanks, Alex

20th June 2005, 09:22 PM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	It's mainly a question of matching the gain of your chip amp with the gain of the preamp so that you don't have too much. Too much gain would give you little useable range on your volume control. Can you tell us what is the gain in your chosen preamp? __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

21st June 2005, 08:36 AM	#3
schmalex diyAudio Member Join Date: May 2005 Location: Seattle, WA, USa	Gain Well, that's a good question. The manual for the GTP-400 doesn't list gain, but it does list Loudness (volume at 9'oclock) - 100Hz.............+5dB 20Hz.............+10dB I assume that's the same as gain, yes?

21st June 2005, 09:51 AM	#4
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	No, they are talking about a boost (optional) of the lower (bass) frequencies when listening at lower volumes. It's not absolutely essential but if you want to do things properly, you will need to know how much the input signal to the pre-amp is increased (gain) before the output to the power amp. I'm not so sure I would trust a hi-fi component supplier that couldn't supply this basic information! I know that you say 'convenience' is an issue here but why not just build your Gainclone, use it and when it is convenient, build a pre-amp? If you build the pre-amp shown on Decibel Dungeon, it has a unity gain so you wouldn't need to change the gain in the power amp section! But if you wanted gain in the pre-amp, you could easily change the gain of the power amp to suit. Having said that, you could probably just buy that pre-amp and tailor the gain of the GC accordingly. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

21st June 2005, 06:18 PM	#6
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	I can't tell from that spec what the gain is! You can alter the gain on the GC by changing the feedback resistor or input resistor, providing that you don't lower the gain below 10 and keep a high enough input impedance. If your heart is set on the pre-amp, put the two together as they come and see how they are together. If you find that it is hard to alter the volume precisely, lower the gain on the GC as described above. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.


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21st June 2005, 09:10 PM	#9
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	OK we want no smart-***** here chaps. Tell us how please! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

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