Parasound HCA-1000 upgrade

[Skorpio]

I have read John Curls suggestions on Parasound amplifier upgrades:

-Higher bias
-Remove parallel capacitors in PSU lines
-More filter capacitance/higher quality

Do anyone have further suggestions or recomandations?

The standard input impedance is 33k, but according to this test it is measured to 48k? Wonder why?

http://www.audioxpress.com/reviews/media/AX601HA.pdf

I would like to increase the input imp to 100-300k range for better interface to passive preamp. It this possible without major problems?
The JFET input is high imp. so the total should be a fixed resistor...or?

[Skorpio]

As I can see inside the imput impedance of 48k consists of a 50k levelpotentiometer on the rear, some cables and a 1M resistor i parallel with a 200pF capacitor (and then the JFET gates)...

It should be simple to remove the potentiometer and get a input impedance of 1M.....

What is the purpose of the 200pF? Is it to ensure that signals above a specific bandwith don't enter the amplifier?

Could it be reduces or removed?

[Skorpio]

I will drive the amplifier with a passive pre (10K Noble potentiometer) and low capacitance cables (37pF/meter)....so the 200pF seems high direct on the input....

Hvad will happen if I remove it completly? Or will it be better to reduce it to 47 or 100pF?

[Skorpio]

Looking at the Parasound A23 (for inspiration) something caught my eye. Look at this picture:

http://www.parasound.com/img/halo/A23_inside_big.jpg


And reading the text from the page:

12 beta-matched 15 amp, 60 MHz bipolar output transistors

It is clear to se that only 8 power transistors is mounted, even though the PCB have space fpr all 12....

An upgrade?

Anybody know the output emitter resistors (type)?

[Skorpio]

The feedback resistor is a standard 1% MF resistor 47k5 value. Would it be a problem replacing this with a 0,5W Vishay 49k9 VTA52 bulk resistor?

I am thinking about the changes value, giving less feedback/more gain...

[IMSTOOPID]

HEY FEDEX KILLED MY AMP..
I WAS THINKING (OR WAS THAT DRINKING) SINCE I USE ONLY BALANCED INPUTS..

I would rip out the input board and bypas the post and all
and just put in my 1:1 jensen transformers.

and change the emiter resistors

[gni]

Looking at the Parasound A23 (for inspiration) something caught my eye. Look at this picture:

12 beta-matched 15 amp, 60 MHz bipolar output transistors my *****!

There be some transistors missing. . . .pays to look under the hood.

Unless they are in the fourth dimension. . . maybe Parasound
should have said: sounds like 12 output transistors but only needs
8 to do the job.

Is it me or do the Halo series kind of look cheap from the rear.

[john curl]

I don't appreciate 'second guessing' my designs. The HCA 1000 is my least expensive design and is quite a bargain, if I say so myself. It is made for home theater use, primarily, and is marginal for quality audio. I use one for home theater reproduction, also powering a subwoofer and A-B rear channels. It does an adequate job, but just barely.
Serious listeners should get the A21 or the JC-1 monoblocks. That is where I put my best effort.
The A23 uses my design, but not my direct input. 2 means 2 ch, 3 means cheaper.

[IMSTOOPID]

JOHN:

I'm not second guessing your work. Just ask Mr. Bybee about the guy in TN who built his 1 production Bybee boxes and filter housings!

What happened is an A23 I had in a triamp system got whacked by Fedex when it was loaned out and shipped back!

The pots are all "scratchy" and and the input source switch got whacked.

I remember from a discussion we had 7-8 yrs ago I did tsome mods to my 1200II's since all my gear was balanced and I needed balanced inputs.. I had heard Jack's amp that you or Carl had modified and loved it.

I know we changed the input resistors for (10K load) the emmiter resistors, some caps and a few minor tweaks. To the life of me I cannot find that info anywhere and the amp died form a lightening strike!

private reply's welcome

Ken Campbell

[john curl]

I see a mixture of opinions and inputs. First, it should be possible to remove the 50K pot. Then the input impedance will be higher. 100pf should be OK, rather than 200pf, but the amp might oscillate, IF you remove all the input capacitance. Actually, you should just leave it alone, and everything will be OK, and you will hardly notice the loading difference.