Is Gainclone Direct Coupled? Appropriate Preamp? - diyAudio
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Old 6th April 2011, 03:59 PM   #1
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Default Is Gainclone Direct Coupled? Appropriate Preamp?

Hi Guys,

I'm new to this forum, and I was thinking about having a gaincloan (3886) built for me for a pair of Jordan Jx92s transmission line speakers I am building. I was also thinking about using a LittleDot headphone amp/preamp in the system, but they tell me not to use it with a direct coupled amplifier. I assume that the gainclone is direct coupled, am I right? Is this because the preamp passes DC, so you shouldn't use it with a direct coupled amp? What should I look for in a preamp to make it suitable for using with a gainclone? What about a gainclone with a simple volume pot incorporated into it?

Thanks, Peter
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Old 6th April 2011, 07:15 PM   #2
AndrewT is offline AndrewT  Scotland
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Simply asking the question tells me you are not ready to adopt Direct Coupled Amplifiers.

The risks must be appreciated and the operator must decide to accept or ameliorate those risks.
Don't go Direct Coupled until you know the risks and how to reduce the chances of damaging your equipment.
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Old 6th April 2011, 07:29 PM   #3
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Quote:
Originally Posted by PeterSc View Post
Hi Guys,

I'm new to this forum, and I was thinking about having a gaincloan (3886) built for me for a pair of Jordan Jx92s transmission line speakers I am building. I was also thinking about using a LittleDot headphone amp/preamp in the system, but they tell me not to use it with a direct coupled amplifier. I assume that the gainclone is direct coupled, am I right? Is this because the preamp passes DC, so you shouldn't use it with a direct coupled amp? What should I look for in a preamp to make it suitable for using with a gainclone? What about a gainclone with a simple volume pot incorporated into it?
If we are referring to Figure 1 in application note: http://www.national.com/ds/LM/LM3886.pdf then yes, LM3886 based gainclone is direct coupled amp. However, nothing gonna prevent you from installing coupling cap at the amp input, and you'll be fine with a LittleDot.
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Old 6th April 2011, 08:11 PM   #4
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Thanks to the Canadian! (Peter Daniel). Your solution is exactly what the guy who would build the amp just emailed me to suggest. Does adding a coupling cap have much significant effect on the performance of the gainclone?

If anyone is willing to answer, I do have remaining questions, as what I'm reading online is somewhat contradictory:

1) I'm reading that most SS amplifiers are direct coupled. Is this true? If it were true, than Andrew's response (above) disqualifies most people from using most amps! Seriously - why is this not more directly dicussed?

2) I'm surmising that direct coupled has no way to block DC. The input cap recommended by PD blocks DC from entering the amp. Where does the DC originate? Source? Preamp? Power supply in the source, preamp or amp?

3) Somebody wrote that if you have a direct coupled amp you should use a direct coupled preamp. I don't see how that would add any filtering of DC to the amp/preamp combination. Does this make any sense?

Thanks, Peter
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Old 6th April 2011, 08:35 PM   #5
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The average gain setting is high enough not to need the colouration of a preamp stage. I went from opamp to valve buffer and back and eventualy came to the conclusion that the best preamp is a potentiometer, or better yet a stepped attenuator.

A low enough frequency signal is as good as DC....
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Old 6th April 2011, 08:45 PM   #6
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the input capacitor is often the most important component in the chain. Easy to hear the effect of a bad/good one.
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Old 6th April 2011, 10:13 PM   #7
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Interesting - cats squirrel. The guy building the amp for me said the the input cap should have no effect on the performance! He suggested using a standard polyprop cap. Should I be aiming higher in this application. I know it's a matter of personal taste, but for a budget amp like a gainclone, any suggestions?
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Old 6th April 2011, 11:50 PM   #8
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If you are on the budget, you have the "replacement" of Blackgate, the bi-polarized Nichicon Muse ES (the green one) it is about 1 Euro the 10uF/35V.

I have that on my LM3875, shunted by 16nF Stiroflex (1.30 Euro)... for the moment, not sure it is good enough !
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Old 7th April 2011, 07:00 AM   #9
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Of coarse if size permits I'd kick his behind for trying to start my amplifier chain trough an electrolytic. Even a cheap polyprop cap would be miles better. I don't think you realy need 10uF though, and that wil be quite big and expensive. 3.3uF should be more than adequate, even lower, especialy if your speakers don't go all that low.

Don't discredit the chipamp out of hand, it sounds a fair bit better than most gear under $200.
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Old 7th April 2011, 09:05 AM   #10
AndrewT is offline AndrewT  Scotland
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Most SS amplifiers are AC coupled.

The output is usually directly coupled to the speaker. The Dual Polarity Power Supply that is almost exclusively used in ClassAB amplifiers allows the output capacitor to be omitted.
I think it is this capacitor you have mistaken for AC or DC coupled.

A DC coupled amplifier amplifies all frequencies with in it's pass band to the same gain as DC signals.

An AC coupled amplifier blocks DC at it's input and has the DC gain set to 1 (+0dB)
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