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direct coupled vs capacitor coupled ?

adeel

adeel

Active Member
hello friends , hope all you are fine and enjoying this beautiful life gifted to us. i want to understand some thing related to direct coupled preamp and power amp.

i see some very good restoration people advising never use a dc preamp with power amp having dc input. what i experimented using dc connection between preamp and power amp give more accurate sound than capacitor coupled switche on . ie more focused , tight , good roll off of high frequency , streched in more accurate limits , etc.

but it gives more heat from power amp.

So some one can suggest if i should use dc connection or not ( provided dc off set is fully accurate ).



and one more thing i see some very good class A preamp so which is better dc preamp or class A preamp?
 
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The key is to make sure your preamp does not have any DC on the output. The only thing the cap is doing electrically is blocking that. If there is no DC to block, there isn't any need for it.

If you do have DC on the preamp output it will create offset at the speaker terminals. If you're getting extra heat, fair guess the preamp is not 0vdc on the output.

might also be worth replacing / upgrading the output coupling cap if you are actually hearing it.


most preamps are class A. For small signals it just doesn't make any sense to be otherwise.
 
thanks gadget73 you gave very good information for my question . i don't know if there is any recording that have direct current (very low hz 0 , 1 or 2 hz) or may be our rc connectors that takes some noise which can be decreased with capacitor decoupling?
well i can now understand how to deal dc preamp with dc input power amp . thanks agaian.
 
I did read in one of my cap coupled amps manual that it can be a benefit to a system that uses an electronic crossover, as the cap can provide protection for the 'naked' speaker it's driving, it even suggested different values of capacitor depending if it's driving tweeters or mids.

As for the DC are you referring to the amps circuit design? A few of my luxmans are 'DC amps' but are still class a/b.
 
Direct couple means no capacitor in signal path . Caps usually produce colouring in sound . But problem with old dc audio systems is they start producing dc offset which may be lots of reasons .
Luxman i think makde lots of single stage amplifiers speciality is duobeta circuit of luxman . I think it uses two feedbacks . But i also see some good denon amp with no negetive feedback .
What you are talking about what i understood is power stage class which can be class AB , class A , class G , class D. I think all these classes in push pull design have no capacitor in speaker out path .
 
gadget73 said:
The key is to make sure your preamp does not have any DC on the output. The only thing the cap is doing electrically is blocking that. If there is no DC to block, there isn't any need for it.

If you do have DC on the preamp output it will create offset at the speaker terminals. If you're getting extra heat, fair guess the preamp is not 0vdc on the output.

might also be worth replacing / upgrading the output coupling cap if you are actually hearing it.


most preamps are class A. For small signals it just doesn't make any sense to be otherwise.
Click to expand...
Another approach would be to get a preamp that has a DC nulling servo if you do not want a capacitor in there. Of course, if you really don't like capacitors, then the servo won't help since the integrator in the servo has what else? a capacitor.
 
DC nulling servo i saw in preamp denon pre-2000z and dap-5500 but both amp required to set dc off set to zero intially . i don't see in sansui au-919 , au717 , au-819 i think in no integrated amp i found this ciruit. what i understand it diminishes direct current from signal path if intially set dc off set to zero.??? i don't understand it deeply.
 
I am not familiar with that equipment, but maybe the DC offset has to be initially manually zeroed because the offset may be too large for the servo to null it, so you do the coarse nulling with the knob and then let the servo monitor and keep the offset trimmed to zero Vdc after that.
 
Plenty of direct coupled amps have caps directly in the signal path, pretty much every one I have worked on have caps directly in the path between the tone bd / pre amp at the input of the amp board.
 
Yeah, most direct coupled amps are only DC within the power amp circuit. Most have a coupling cap in front of the amp to prevent any dc at the input from getting to the speaker. Its an amp with bandwidth down to DC so dc in causes dc out.
 
Yess friends you are 100% right ,
integrated which i restored , upgraded for exaple
sansui
Au-819, au717 , au519, nec a-10 and preamp denon dap-5500
all description says they are dc amps and preamp
but i found lots of caps in signal path but at the output no cap i found so
what i understand dc means is no cap in output and in dc power amp section i don't see cap in single path .
 
I really don't have any problem using a capacitor as a coupling device if the cap is well made using good materials and is large enough to pass the lowest frequencies required. Use a cap that is no larger in value than needed. In the case of tube amps, they are a necessity for the majority of the amplifiers; can't get around that. Go with parts made by reputable makers and be very skeptical tweako magical ones.
 
hi egellings you advised to use cap large enough to pass lowest frequencies can you plz explain with example ???
 
capacitors into a load form a high pass filter. Lets say your power amp has a 10k input load. If you use a 1uf capacitor at the output of the preamp into a 10k load, the -3db point is 16 hz but its going to have a fair bit of phase shift at that point. You probably want at least a 10uf cap to work into a 10k load. The smaller the cap or the lower the load resistance, the higher the -3db point is.

Coupling Capacitor Calculator by V-Cap

Coupling capacitor calculator that calculates optimal coupling cap values based on the input impedence of load, and the desired low frequency response you desire
www.v-cap.com

will let you calculate it. There is a math formula that involves funny symbols that I don't know how to type but using a computer to compute things is easier.
 
Thanks gadget73 . I want to undetstand some thing , in preamp flat board we see non polar caps of value 10uf , 22uf , 47uf , 100uf in signal path can we increase that value?
I mean if an preamp input having 10uf BP elecrolytic cap of nichichicon muse es can i upgrade it to 22uf ? If no impact than which is better option 10uf or 22uf ? Or i should replace it with 10uf crossover polypropylene sonic cap 10uf 250v ?(provided having space to fit it) Too many question but i want to understand. thanks .
 
This problem, as well as problems with ground loops and other nosie, reinforce my belief that high-quality integrated amps get a bad rap from audiophiles. A competently-designed integrated amp makes a lot of sense. Short signal paths in one box can reduce noise, and the separate power supply canard is dispelled when you realize that preamps don't use much power. Yes, crappy equipment is integrated, but that doesn't mean that all integrated amps are crappy.
 
In theory they are a lovely idea, in reality some are several turds in one box. This differs from setups with several boxes with one turd each :)

But yeah generalizations don't cover everything
 
Many are a bit of a comprise. But there are some very nice integrated amps. Marantz made some in the 70's where they took they're TOTL preamps and power amps and put them together as integrated amps.
 
My stuff is a mix of cap-coupled and direct (Pass F5 with the MOSFETs connected directly to the speakers). Proper selection of the cap type and value (as @gadget73 said) is key to getting the best sound while protecting your speaker voice coils from being melted by DC.
 
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