such a thing as AC or DC stereo amps?

danger boy

My friend asked me about a DC amp. saying he's heard they are better than AC amps. What I have never heard of either.

Ok, he just bought a vintage amp.. and it says.. model number XXXXDC and another amp he has by the same brand.. mitsubishi says model number XXXXAC

what the hell is a DC amp? if such a thing exists?

engtaz

1 is goes to the car the other is for the house. LOL

W WALDECKER

danger boy wrote: »

My friend asked me about a DC amp. saying he's heard they are better than AC amps. What I have never heard of either.

Ok, he just bought a vintage amp.. and it says.. model number XXXXDC and another amp he has by the same brand.. mitsubishi says model number XXXXAC

what the hell is a DC amp? if such a thing exists?

there are AC coupled and DC coupled power amplifiers. Thanks....WCW III

Ricardo

From the Soundstage review of the Belles 350 ref.:

"The 350A Reference is also AC coupled, while the 350A was DC coupled. The difference is essentially safety for your system. An AC-coupled amp will not pass DC to the loudspeakers, while a DC-coupled amp will if any is presented at the input. Under normal conditions, this should not be an issue, but, as we know, sometimes stuff happens. There are also sonic considerations here. AC coupling compromises the sound and may limit bass extension in the bottom octave because a capacitor has to be inserted in the signal path to block DC from passing through the amp to the outputs. Dave Belles managed to insert a capacitor that doesn’t limit the deep bass (detected by ear or by SPL meter using the DC-coupled 350A as a reference point). Thus, there’s no audible harm to the audio signal from the extra capacitor that I can detect."

heiney9

From a purely sonic POV DC or direct coupled is the way to go. If you need to couple the input stage to the output stage and it's not a direct coupled design you can use a transformer instead of a capacitor as it doesn't degrade/change the sound as much. Transformer coupled amps (SS anyways) are few and far between and to implement it properly is quite costly.

Usually in an older amp one of the reasons it can sound like **** is because the coupling caps are on the way out plus many manufacturers use electrolytic caps, and coupling caps are in the signal path so even changing perfectly good operating electrolyics with a different type of cap is preferred. This is where Black Gate Caps come into play for many "upgrade" paths in an amplifier.

H9

P.s. This one of the reasons I like the first generation Adcom amps. All the later generation amps are capacitor coupled. I will be checking the DC offset on my amp very soon to make sure there are no DC issues that could harm my SDA's.

heiney9

DC-coupled
In terms of analog circuitry, electronics that are DC-coupled have their components connected directly together without any coupling capacitors. As capacitors tend to remove certain frequencies, coupling the circuitry without capacitors in-line allows the full spectrum of sound frequencies to pass through unfettered. This can be advantageous as many capacitors severely limit the passage of low frequencies, resulting in poor low-frequency response. Manufacturers such as Solid State Logic often use DC-coupled circuitry for this reason. As capacitors wear out over time, many devices will experience a decline in performance. By building equipment without capacitors in the circuit, a manufacturer ensures that will have the same frequency response and essentially sound the same years after its initial purchase. To illustrate this point, we'll use a SSL DC-coupled XL 9000 K console as an example. Since no capacitors that can fail are in the signal path, channel 1 will sound the same as channel 32 (or any other channel, for that matter) year in and year out. DC-coupled equipment maintains a linear-phase relationship across all channels, further illustrating the advantage of this method of creating circuits. DC-coupled circuitry also tends to be utilized in audiophile and high-end recording equipment, as it tends to provide the "purest" sound. Capacitors, of course, still play a large role in the construction of electronic equipment. In applications where DC voltage may damage the circuit, capacitors are placed in the signal path as a means of protecting transistors from being burned up by the DC current. Capacitors essentially condition the incoming signal by rejecting unwanted DC voltage. DC voltage is also typically undesirable in audio signals as it can cause distortion later in the signal path.

From Sweetwater audio glossary

jakelm

Is there a key design inside the amp that would indicate a DC amp? Or is as simple as tracing backwards from the speaker output and finding a cap inline within the output.

On my older DH-200 Halfer, I went through the whole board, didnt see any caps inline of the output.

heiney9

jakelm wrote: »

Is there a key design inside the amp that would indicate a DC amp? Or is as simple as going backwords from the speaker output and finding a cap inline.

On my older DH-200 Halfer, on the board, what would I look for?

Yes, try going backwards, depending on the amp there maybe other caps in teh signal path so you'd really need to pay attention to where the input and output sections come together. I hate reading schematics because I'm no good at it.

Chances are if you have the original literature it might say in there. Without seeing the schematic or picture of the board it's hard to tell. Check the DIYaudio forum there maybe a post there with info.

H9

jakelm

Thanks H9. I edited my post. In not remembering any inline caps withing the output.

If a cap was found inline with the output, couldnt it just be removed?

heiney9

jakelm wrote: »

Thanks H9. I edited my post. In not remembering any inline caps withing the output.

If a cap was found inline with the output, couldnt it just be removed?

No, it would be an open circuit and it's not just a matter of running a jumper in place of the capacitor. The amplifer design is centered around the original design. To improve it you could use a better coupling cap like Black Gate, etc. or if you are really good you could probably redesign the entire amp.

H9

jakelm

Not sure

heiney9

^^Sorry I don't speak Chinese. . I'll have a close look later and see if I can help you out if no one else has by that time.

H9

jakelm

heiney9 wrote: »

^^Sorry I don't speak Chinese. . I'll have a close look later and see if I can help you out if no one else has by that time.

H9

LOL.... I had to throw that schem of my amp. I couldnt help it.


You look at the board and the amp, very simple design. But you look at that schem and you think "what the %^&*?"


But I would put $10 on no, this amp is not a DC amp.

danger boy

my friend thanks you for the info..
peace. DB

Systems

jakelm wrote: »

Not sure

Jakelm,the amp in the schem has an input coupling cap it is marked C1.

jakelm

GV#27 wrote: »

Jakelm,the amp in the schem has an input coupling cap it is marked C1.

As old as that amp is, do you think it needs to be changed? Or is it in a position where it does effect the SQ?

heiney9

jakelm wrote: »

As old as that amp is, do you think it needs to be changed? Or is it in a position where it does effect the SQ?

It's a coupling cap so it's in the signal path. I'm sure replacing them will improve the sound. What issue are you having? FYI, I just read Black Gate finally closed it's doors as of August 2007.

H9

jakelm

Its a 10 mfd non polorized electrolytic (C1). Im sure its on both left and right input. I dont see anything on the output. I think I might just go ahaed and change it, beings it is an electrolytic cap. They do degrade fairly quickly.

This is the schem for just one channel, both channels are identical.

Problems are, little to no deep bottm end, and very crispy (harsh) highs. This amp has been on Polk 7 for 20 years and has always sounded great, but the past few years I have noticed a steep decline in sound quality.

I am especially gad someone mentions DC/AC amps. Which is why I asked about my amp.

jakelm

heiney9 wrote: »

It's a coupling cap so it's in the signal path. I'm sure replacing them will improve the sound. What issue are you having? FYI, I just read Black Gate finally closed it's doors as of August 2007.

H9

Well Black gate made very good caps. I guess its back to PExpress

jakelm

I think while I'm buying parts I will change out the big 10,000 uF 75v Electrolytic caps as well. They look easy enough to change.

heiney9

jakelm wrote: »

Well Black gate made very good caps. I guess its back to PExpress

I'm sure some outlets still have some stock but I'm guessing you'd have to give a kidney to get one .

Not sure what a good rpelacement cap would be. Some others, like F1, who have done some modding might chime in.

H9

jakelm

PE doesnt have anything close to the size of the huge 10,000uf caps. Might check mouser.

The 10uF 16v shouldnt be too hard to find, unlike the 10,000.

I might start a new thread in the DIY. I think I highjacked this thread long enough. Sooo sorry.

Systems

jakelm wrote: »

Its a 10 mfd non polorized electrolytic (C1). Im sure its on both left and right input. I dont see anything on the output. I think I might just go ahaed and change it, beings it is an electrolytic cap. They do degrade fairly quickly.

Problems are, little to no deep bottm end, and very crispy (harsh) highs. This amp has been on Polk 7 for 20 years and has always sounded great, but the past few years I have noticed a steep decline in sound quality.

I am especially gad someone mentions DC/AC amps. Which is why I asked about my amp.

I would replace C1 with a 10uf film type cap(poly prop ,mylar etc) if you have the space to fit it.
Coupling Caps are not needed at the outputs of solid state push pull amplifiers.

Are C5,C6 in the feedback loop electrolytic types? Bad ones here can reduce the bass response.

Definately I would replaced the big filter caps.It looks like they are the old can type and may be hard to source.Otherwise you will have to get some PC tab mount types and find away to secure them into the chassis.I have a bunch of spares(freebies)of this type at work if you are interested.

jakelm

Thank you GV. Your saying you have the tab mount type? If you think that would work and if you have the right size, I'll take you up on that offer. I will replace both c1's as you recommended.

C5 is electrolytic, C6 is Mica.