Zu Soul Superfly

Gopher:

I mentioned to you on 7/06/2010 about upgrading your F1 to the F1J. I've read here since that you are going to do so. One potentially huge difference (problem?) for you, especially if you are using a tube pre: the input impedance for the F1 is 100K ohms single-ended and the F1J is 15K ohms (note at www.firstwatt.com under products it says 100K and 10K respectively, clicking on F1J it says 15K single-ended). Rule of thumb (from conrad-johnson): pre output impedance < 10% x amp input impedance, i.e. your pre must be less than 1500 ohms or you will probably lose some of the low-end frequency. You might want to send an email to Nelson Pass at nelson@passlabs.com and discuss this, especially if your pre has an output impedance greater than 1500. I'm anxious to hear what you think of the Soul/F1J combo, if and when you finally upgrade.

Regiggey,

Thanks for the concern. I asked my dealer who in turn asked the designer of my preamp and he said the following:

"I do not have the spec. of this F1J machine, but I guess it should be some SS power amp. The MM Pre has a cathode follower output stage, it should push this F1J fine."

A lot of the tech talk is lost on me, but I am concerned now reading this rule of thumb as the published specs are of my pre are:

Input Impedance: 100K Ohm
Output Impedance: 700 Ohm

Hmm, I already bought the damn Semisouth R100s and got an audio buddy to agree to do the update for me... Is this likely to be a poor match?

The rule of thumb is 10:1, that is, the input impedance is 10x the output impedance between the two components. I've also heard the rule of thumb is 20:1, but hey, it's a rule of thumb right, so the truth is probably somewhere in the middle, somewhere between 10:1 and 20:1.

MM Preamplifer has 700 ohms output impedance
700 ohms * 20 = 14Kohms
F1J amplifier has 15Kohms input impedance

14K < 15K

So you should be good.

Gopher:

You will not have a problem. Your are telling me that your pre output is 700 ohm which is less than the critical number of 1500 ohms for the F1J. 10% x 15000 ohms = 1500 ohms (F1J input single-ended impedance is 15K ohms (K=1000) or 15000 ohms. 700 ohms < 1500 ohms, so you will have no problem. Now, I'm assuming that the person who will be updating your F1 will be doing what Mr. Pass does when he updates a F1. To be safe, have your guy contact Mr. Pass via email and discuss it with him.

OK, I think I mis-read you and was concerned about input impedance.

Nelson Pass was goodly enough to post step by step instructions for the conversion on diyaudio as he does with all the Firstwatt deigns.

Step by step:

1) You match up a pair of SemiSouth R100's or equivalent and
drop them in for Q1 and Q2

2) You eliminate the Zener diodes

3) Raise the input cap values to 10 uF

4) Replace R1 through R4 with 22 ohms

5) Replace R5-R8 with 47K ohms

6) Set the bias pot to +14V on the Drains

All Greek to me, but I sourced the R100s from a group buy on diyaudio and have a competent friend willing to do the update.

Off topic--or back on topic, rather. These Soul Superflys have been the single most meaningful upgrade to my system. I'm listening many hours daily virtually every day. I'm really confused as to why Zu speakers have been so controversial over the years. Based on what I read on forums, I really expected them to have a weird sound but they sound very right to me, making real music.

The other owners really need to chime in! I feel like I'm all alone here.