Cap Coupled vs. DC

[onplane]

Mike,

There is another interesting "transition" amp. I built one as an Eico kit. This amp was direct coupled output, but had an interstage transformer.

The first AR amp was very similar in design.

What makes these amps unusual is the transformers were quite modest in size since they had no real power to handle. Nevertheless, those transformers would add harmonics similar to the output transformers in tube amps.

My guess is these ss amps are the closest ss amps to sounding like tube amps.

Regards,
Jerry

 

[M Jarve]

The h/k 720 I mentioned above is sort of indicative of early solid-state receivers. It is, for all intents, a Festival series tube receiver, but with transistors used almost as drop-in replacements for the tubes, and with appropriately lower voltages. The output transformers are identical in size to the A224 amp that preceded it. The stage between the tone amp and the power amp is capacitor coupled on this unit, which would be the typical design for nearly every receiver built after.

 

[mhardy6647]

A photo of the "720" to which you refer would be very interesting.

Many (if not most) very early SS designs used IT's (interstage transformers), possibly due to discrepancies in impedance characteristics of available (germanium?) transistors at the time?

The early transistor era was not a pretty thing. For example, good RF amplifier transistors were slow in coming, thus the prolifieration of early SS tuners and receivers with RCA "Nuvistor" tubes in the front end. Some, I believe, were quite good.

In the late-1950's, hybrid car radio designs with tubes in the front end, but with transistor outputs appeared. These presumably eliminated the need for an output transformer. The development of "space charge" tubes that used very low plate voltages (ca. 12 VDC) also eliminated the need for a HV "vibrator" type of power supply for car radios. Today, these low-voltage tubes are cheap and plentiful.
HiFi headphone guru Pete Millet put them to good use in a hybrid headphone amp/preamp that uses a wall wart for power (no HV)!

http://www.pmillett.com/hybrid_head.htm

http://www.duntemann.com/12vtubes/12vtubesindex.htm

 

[sanyofreak]

An interesting, early evolutionary step between "single-ended" SS power supplies (and cap-coupled outputs) and the later ubiquitous, separate positive and negative DC rails complementary design were the cap-coupled, quasi-complementary SS amplifiers. Scott made several of these. Reports from the field have indicated that they can sound very good indeed.
http://db.audioasylum.com/cgi/m.mpl?forum=vintage&n=132285&highlight=Scott+LK-60&r=&session=
http://www.lacieg2s.ca/w3terra/ols/scott260.htm

"complementary" and "Quasi-complementary" are terms that describe an amps section (usually output stage) and are not tied to the presence or absense of a coupling device such as a transformer or capacitor. The Scott unit that You describe, is still cap coupled and this tells the tale more so than it's output stage configuration.
Complementary outputs use opposite polarity devices to handle (in class B fasion) the seperate halves of the signal, for instance an NPN and PNP transistor combine to make a complementary stage.(or N channel and P channel FET's for example as well)
Quasi-complementary outputs (also known as "totem-pole" outputs) utilize all the same polarity devices in the stage to handle the positive and negative swing of the signal (For example all NPN's). I have seen both of these arrangements used in both DC and cap coupled SS amps, and again, dont necessarily determine if a coupling device is there or not.

I'm a fan of the DC arrangement, but a Famous amp designer that I respect has some good things to say about DC - and that it is NOT important to have an amp that can pass or amplify it. Many of You may have heard or seen these points which are very valid.
My speakers cannot reproduce DC
My ears cannot hear DC
DC cannot be recorded even with an SACD
No musical instrument can play a DC note, nor can a singer sing DC.
No microphone can pick up DC
And on and on. Kind of smart A$$, but You get the point.

That said, I still like the sound of DC coupled SS amps the best. And - there are good designs and fine examples from all schools of construction, and I'm sure some poor ones too.

 

[mhardy6647]

Complementary outputs use opposite polarity devices to handle (in class B fasion) the seperate halves of the signal, for instance an NPN and PNP transistor combine to make a complementary stage.(or N channel and P channel FET's for example as well)
Quasi-complementary outputs (also known as "totem-pole" outputs) utilize all the same polarity devices in the stage to handle the positive and negative swing of the signal (For example all NPN's). I have seen both of these arrangements used in both DC and cap coupled SS amps, and again, dont necessarily determine if a coupling device is there or not.

Absolutely so, but since the signal isn't swinging around 0V, you need a blocking cap on the output. That's all I was referring to.


As far as the relevance of DC... since real musical waveforms are asymmetrical (even, sometimes over very short timeframes), there is a quasi-DC component to real musical waveforms, even if there is none to a (n unbiased) sine wave.

Still, the best sounding amps at my house have not only caps in the signal path... they have vacuum tubes too.

 

[SteveP]

Just to sidetrack the thread for a bit, I've also seen some amps where they speak about 'DC coupled' and 'cap coupled' when they refer to the INPUT section of the amp, ie. they have the ability to switch a cap in or out of the circuit at the input side of the amp. This would be useful if there was a bit of DC on the output of your preamp or source component if you hooked that directly up to the amp.

Steve.

 

[mhardy6647]

I think that the "DC" sobriquet in an integrated amp or receiver is generally taken to mean no capacitors in the signal path. Of course, that's not always the case. No blocking cap in the input can leave you exposed to DC bias in a source component.

 

[oldtubes]

My first receiver was an ECIO 3570 designed in 1967. It has a quasi-complementary cap-coupled amplifier. It is only 15 watts/channel but it has one of the widest frequency response in my house at 5hz to 100khz +/- 1.5 db and does a great job on a 20hz square wave. The frequency response is in the same league as my Sansui BA-2000. The EICO is not the loudest playing receiver but it sure sounds sweet at lower volumes.