Quickie & Low Impedance Power Amp

[danielehn]

Hi,

Have been very happy with my Quickie (with PJCCS) pre-amp now for some time.

I recently purchased a "Holfi Power 8" power amplifier which in it self is
a strange beast. On the circuit board of this amp there is a dip switch for
setting it to 800Ohm input impedance.

According to other owners (in forums etc) setting it to 800Ohm input impedance
gives the best performance.

Therefore I want to flip the switch

However I don't think that the Quickie wants to go that low (sounds like headphone impedance)

Can I modify my Quickie to go that low?
(i.e. head-phone modification)

And if so do lose any of the lovely sound it produces today?

Found this in a forum (does it help) -->

"Some years ago I talked with Peter Holstein from Holfi. He uses a 75 ohm output impendence from there preamps and 800 ohm input impedance in there power amplifiers, for the same reason, though they don

[Grainger49]

You will not harm the Quickie playing it into an 800 ohm input impedance.  It will probably roll off the deep bass.  That is what happens as the output capacitors and the input resistors form a high pass circuit.  Tubed peamps almost always use an output capacitor.  A few tubed preamps use an output transformer.  So there is probably a transformer that would relieve this problem, but I don't know which one.

I would say try it and see what you hear.  It could be that the low notes lost are not reproduced in your room/system/speakers anyway.  So try it.

Just to be sure I understand you, the posters who say the amp sounds best are using a bunch of different preamps, right?  It is the reaction of the preamp output impedance (including the output cap) that affects this low frequency roll off.

[danielehn]

Thanks for the answer Grainger49

Sorry if my question is stupid. Is it also harmless for the poweramplifier ?
(something fried or superloud)

"the posters who say the amp sounds best are using a bunch of different preamps, right?"

The pre-amp this poweramp is supposed to be paired with has the option to be set to "low impedance" mode. They are probably refering to tests running that pre-amp in the more generic mode vs the "low impedance" mode.

[danielehn]

Found some information regarding another power Amp from Holfi -->

"Recommended output impedance from preamp: 150Ohms - 1kOhms "

(Builds on the same principles as the one I have)

[Grainger49]

Yes, it is harmless to the power amp.  I'll assume that it is solid state.  Give the Quickie 30 seconds to warm up and all DC on the output to dissipate before turning on the power amp.  Otherwise you will get a thump.  If the power amp is capable of amplifying DC the turn on thump could damage your speakers.  Most power amps will not amplify DC.

So with what you have now said the power amp may not (EDIT:) sound best reproduce all audible low notes with the low impedance input.  But I stand by trying it, especially if the amp can have the input impedance switched while it is on.  That would allow you to listen to each input impedance setting with a minimum of bother.

[danielehn]

I am trying to say the opposite. That according to the manufacturer and users it does sound at it's best with low impedance input

Here is a review from an owner describing his experiences -->

http://www.audioreview.com/cat/amplification/amplifiers/holfi/power-8/prd_115831_1583crx.aspx

Last note: there is an input impedance switch (on the circuit board). Holfi recommends the 800 Ohm(!!) positionand so do I. It does add much detail/musicality, but it also
puts more demands on your pre-amp to drive it. Holfi's own pre-amps are built for it, and my Metavox didn't have any problems with it - but some might not. If you like the Power-8, and you can't drive it in low-impedance mode - consider changing pre-amp, because you'll be losing out...

[danielehn]

Sry for being a bit messy in my posting..

[Grainger49]

Daniel,

I didn't word that well.  What I meant was that you might lose some bass with the low impedance input (as said in my first response).  I'll modify that post to be more explicit.  

I acknowledge that the amp manufacturer and many/most users find it to be at its best with a low impedance setting on the amp's input.    

The demand on the preamp is higher current needed through this low impedance, Ohm's Law and all that stuff.  I have calculated the roll off frequency of 800 ohms and it is 90 Hz.  Try it, as a matter of fact, if the amp sounds best at 800, start there and go up till you do not hear a difference.  Then go back to the lowest impedance that sounds good.

[danielehn]

You were spot on with the bass disappearing . The mids are clearer though.

[Grainger49]

Once you filter out the power draining bass the mids and highs would be more prominent. 

Did you try higher and higher impedances?

[Paul Joppa]

Like most tube preamps, Quickie cannot drive a low impedance load well. It is rated for 8K minimum; below that the distortion and headroom become increasingly compromised. If the power amp input is so sensitive that neither headroom nor distortion at maximum output are important, then you can substitute a larger capacitor; I would recommend at least 25uF. Have some fun, give it a try!

There are many 10K:600 (or 8K:500) transformers on the market which will give a low output impedance. You do lose headroom of course; this ratio reduces the output to 1/4 of nominal. A pair of decent transformers would normally cost more than the Quickie, but Edcor makes some pretty fair ones suitable for experimenting with and around $15.

[danielehn]

I am probably fooling myself when playing it in the low impedance setting. After switching back and forth a couple of times its very clear that something is missing.I better keep it at the generic setting which I were very happy with before I learned about there being a switch inside the amplifier . Thanks for the help

[danielehn]

Must be something wrong with me! Even though I were perfectly happy with everything. As soon as I read about it being able to sound a tiny bit better I go bananas and almost take my whole system apart.

[mortron]

I have a Denon POA2400A power amp that is rated at 25kOhms input impedance. Will I have problems with the Quickie as a preamp? I read mention of 8kOhms being the minimum. Is this correct? Thanks.

[Doc B.]

Yes, it will be fine running into a 25K ohm load.