Matching a commercial preamp - adcom

[schmalex]

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]

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]

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?

 

[Nuuk]

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]

Gain

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]

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]

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

Why?

It appears to be x6.45

 

[carlosfm]

It appears to be x6.45

x6.45161290322580...

High gain...

 

[Nuuk]

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

 

[carlosfm]

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

2V / 310mv.

 

[Nuuk]

2V / 310mv.

Good man - you are re-elected to the Royal Order of Decibel Dungeon Knighthood!

Is the gain high because it is also having to amplify a smaller signal from the phono input?

 

[carlosfm]

Is the gain high because it is also having to amplify a smaller signal from the phono input?

No, it is high to match their power amps.
There's no standard here.
It is good to have some extra gain on the pre and lower gain on the power amp.
For LM power op-amp chips it's high, because they are stable at 10x gain or higher.
It is a good match for OPA chips, at ~10x gain.

 

[Nuuk]

For LM power op-amp chips it's high, because they are stable at 10x gain or higher.

So it would pay schmalex to reduce the gain of his GC. I'm not sure what values the Brian GT kits use these days but it should be possible to reduce the gain down to something just over 10.

 

[carlosfm]

So it would pay schmalex to reduce the gain of his GC. I'm not sure what values the Brian GT kits use these days but it should be possible to reduce the gain down to something just over 10.

Of course it is possible.
It all depends on the sensitivity of his peakers, assuming that his sources have "normal" line-level.
64.5x total gain (assuming a borderline of 10x on the power amp, not recommended) must be ok for speakers up to around 88~89db sensitivity.
Higher than this and the volume pot will be too sensitive, hard to adjust the right volume.
I like a little more control over the volume, but that's me.

Btw I have a total gain of ~65x, but my speakers are 86~87db.
For me this is fine, if my speakers were more sensitive I would go for lower gain.

 

[schmalex]

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.

Wow, you guys are helpful. Given the instructions above, I have two questions:

Has Brian listed the gain anywhere on the website? I can't seem to find it, but then again, I couldn't find the gain on preamp specs either. Carlos and analog_sa seem to have sharper eyes.

If I do want to change the gain in the amp kit, I assume this is simply a matter of installing a different resistor onto the PCB than the piece included in the kit. Any reason I should replace the feedback rather than the input?

 

[carlosfm]

If I do want to change the gain in the amp kit, I assume this is simply a matter of installing a different resistor onto the PCB than the piece included in the kit. Any reason I should replace the feedback rather than the input?

On a non-inverting implementation, to change the gain you can swap the feedback resistor, or the other one that goes from the inverting input to ground.
What resistors do you have there?
The usual gain for a power amp with LM chips is 20~22x.
As an integrated, with a volume pot (passive) I don't know what gain Brian is using or recommending, but it must be around 35~40x?

 

[schmalex]

sensitivity

Of course it is possible.
It all depends on the sensitivity of his peakers, assuming that his sources have "normal" line-level.

The sources are an old Kenwood T-table and a Yamaha CD player - Pretty "normal" I would assume. The speakers are DIY Seas 27tffc/L15s with a powered sub. The completed speakers didn't come with a manual... big suprise... I made them myself. The tweeter is rated at 91 dB and the woofer at 86 dB. I'm not sure what that means the overall sensitivity is? I guesst the woofer is padded to meet the tweet. Is that right? Please correct me if I'm way out. Either way, not exactly efficient. Thanks.

 

[carlosfm]

Re: sensitivity

The tweeter is rated at 91 dB and the woofer at 86 dB. I'm not sure what that means the overall sensitivity is? I guesst the woofer is padded to meet the tweet.

No, the tweeter is attenuated to meet the woofer.
Sensitivity is low.
You may use this pre, changing the amp for ~12x gain.
I would not go lower than ~15x, though.
It may also be very easy to change the gain of the pre.
Why don't you try to get the service manual of the pre before buying it?

 

[schmalex]

Gain

Thanks everyone, this has been most enlightening. I'll right BrianGT about this issue and see if he can talk me through adjusting the gain on the kit. I'll keep everyone updated.