Been a while, and I have a question. (long)

[mjarve]

It's been a while since I last posted, but it does not seem I missed too much (nor have I had much to contribute).

In the mean time, I've still continued to collect and restore older h/k gear, and I am getting a nice collection.

In fact, it is because of my latest project that I am posting today. I am finishing the restoration of a 1969-70 harman/kardon Eight Twenty (820) receiver. It was generously given to me by the member of another forum I frequent, and he performed his own battery of tests and adjustments to it recently. He even went so far as to include copies of 1970 "Audio" and "High Fidelity" reviews of this unit, as well as an A.G. Tannenbaum service manual!

In an accompanying letter, he noted the particularly low output of the amp section in the bass region.

Here is the frequency vs. output power table he included from a recent test: (bass portion)

Frequency        Volts RMS into 8 ohms        Watts, average (RMS)
------------------------------------------------------------------
20                          8.5                         9
30                         11.5                        16.5
40                         12.6                        19.8
50                         13                          21.1
100                        13.6                        23.1

He found it odd to have such a very low output (and so suddenly) in the <30Hz range for a receiver known for its wide bandwidth. It also did not concur with the Audio and High Fidelity test reports. His explanation is that the output coupling caps may be way out of spec, and maybe the power supply cap as well. I’ve never had experience with solid state capacitor coupled amps before, and a quick search through the site has educated me a little, but it does did not answer the questions I have specifically.

These questions are:

1) The stock output caps are 35v / 1,000ufd units. It is my understanding that for best bass performance, the value should be a minimum of 4,700ufd of capacitance, with 10,000 being ideal. Now for someone trying to get by on the cheap, could I use a pair of NOS 55v / 23,000ufd capacitors I have on hand, or would this be overkill? I also have some 71v / 10,000udf units as well, but they are currently installed on another unit.

2) Does it really matter the type (aside for it being electrolytic, and of sufficient voltage) of cap I use (again, trying to get by cheap), or do I need to use something blessed by the pope? The units installed in there now seem to be identical to other 35v / 1,000ufd caps used in other areas.

3) While researching, I came across a suggestion by one person to use a film cap in parallel with the electrolytic. However, there did not seem to be any follow up response stating whether it was a good idea or not, or if there was any real benefit to that approach.

Thank you in advance for anything you may be able to teach me.

[destroyer X]

Something around 6800uf or a little up will be nice to use as output.
Use a 100N poliester capacitor in parallel, as electrolitic condenser are constructed in a coil shape.... and this made resistance to hi frequency flow. The 100N capacitor will be represent a very low resistance to hi frequencies crossing it.
In the power supply, do not use too big condensers 15000 uf plus 15000uf to 50 watts channel amplifier can be even more than enougth.

If your amplifier produce lower power...you can reduce.... 10 watts each channel, 4700 uf plus 4700uf is more than enougth.

But if you have one "speaker melter" amplifier.....those 300 Watts units...go to 40000uf plus 40000uf.

Those things have some ampere to microfarad ratio calculation formula...but looking at schematics you will have realistic informs, based in Price/Size and a good value not to cause power losses.

A good idea, if the unit is old, is to substitute all electrolitic condenser you find.

Check for oxide in fuses and fuse holders too.

Check the on-off switch...sometimes appear resistance there...panel lamps flicks showing you that kind of problems.

Power input condenser is very important too, related losses of bass....beeing electrolitic can dry and loss some capacitance, showing lower resistance too...some leakage.

regards and good luck!

Carlos

[mjarve]

Thank you for the information. I knew there had to be a practical cut-off somewhere.

The power supply in this unit uses a single 58v DC rail, and a single 63v / 2,200ufd main power capacitor. It seems to be a very clean design with a class B quasi-complementary amp built around 4 Hitachi 2SC1030 output transistors.

The overall sound right now is very tube-like, but the sudden loss of power in the low frequencies is noticeable.

[destroyer X]

Because it is a very old design, and using electrolitic condenser in the output.

Many people turns crazy when we say that sometimes using this coupling the audio results turns very nice, and some speaker crossover factories sometimes use them without make too much noise, to avoid the "purists" of hi-techonology.

The circuit is too much simple.... and the unballance of bass made you perceive better the other frequencies.

I strongly believe in you, as i had some experience very near your experience.

regards,

Carlos

[djk]

http://www.membres.lycos.fr/sst00940/a401schem.gif

See C307?

There will be a similar one in your feedback loop. These are frequently overlooked. Cap dries up, no bass.

How big does the output cap need to be?

2,000µF is 10hz for 8 ohms (and remember older caps had a +100%/-10% rating, so it migh even be closer to 5hz).

Some designs have these in the feedback loop so at less than full power they sound like they go to DC.

A 10µF film cap in parallel helps the HF.

I would bump the main rail cap to the sum of the output caps (remember the output cap is the sole source of current on a negative excursion, positive comes out of the main cap).

I bypass all electrolytics, I like the sound better that way.

If you like a 'warm' sound don't bypass.

[destroyer X]

In parallel with the electrolitic makes enormous differences that you can hear easy.

Carlos

[mjarve]

I've installed a pair of 35v / 6,800ufd caps for the outputs, and one 71v / 10,000ufd cap for the power supply (orginals, again, were 35v / 1,000ufd output caps, and a 63v / 2,200ufd for power). I still need to dig up some bypass caps, as they will be needed, due to dimished high-frequency response.

All of the original caps were Elna brand. The replacement power supply cap is made by "Nippon Chemi-Con", and the output caps are made by "Matsushita Industrial" (Panasonic?). I simply pulled these from parts units I have laying around.

I have the Eight Twenty connected to a pair of "The Voice of Music" model 65 speakers, and the treble control has to be turned up almost all the way for good treble response. However, bass response has improved drastically! Also, the power transformer runs cooler than it did. After I install the parallel bypass caps, I think I'll have called it a success.

One more question, though: Should the 10ufd bypass caps be bi-polar, like in a crossover, or should they be standard, polarized caps? Also, I presume they should be of equal or higher voltage than the other output caps, right?

Thank you again,

[john curl]

You have done OK. I am amazed that they used such small value caps in the first place.

[EchoWars]

One more question, though: Should the 10ufd bypass caps be bi-polar, like in a crossover, or should they be standard, polarized caps?

Film cap...250V is common. I regularly use Panasonic 2.2µf metal polypropylene for this task.