306: Two times upgrade success!

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21-Aug-2015 02:14 AM

Fabian

Fabian

Posts: 102

Just want to leave a quick note that today I added the finishing touch to revising and upgrading my two 306 power amplifiers using DADA electronics kits. All went well and smoothly and I had a lot of fun doing this.

Running a 2-way active setup with a MiniDSP crossover the performance surely is better than before. As always when I have the feeling of a real improvement in reproduction quality, I cannot say that anything has been "added" to the sound. It's rather that something distracting has been eliminated.

Both units have been in obviously unserviced original state. One dates from 1991 and the other from 1994, so it really was about time. Some more detail and photo links to follow soon.

Next up will be the revision of my two 34 pre amps. Did I say two? Why do I need two of them in my active system? Simple answer is, I don't. Got the second one cheap enough so I couldn't resist buying it. One of them will get EJP's deluxe update kit, the other one the standard kit. I suspect the work on the 34 to be a little more fiddly than on the robust 306, but lets see.

Thanks to Stefaan, Joost and last but not least Esmond for keeping this thing going.

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21-Aug-2015 05:52 PM

Marcel

Marcel

Posts: 21

Fabian,

What do you mean with EJP deluxe update kit. What is the difference between the standard Dada Quad 34 update kit?

Marcel

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21-Aug-2015 08:45 PM

Fabian

Fabian

Posts: 102

Marcel, looks like the kit is currently not listed. The content of the deluxe kit is the same as mentioned in the overhaul & update service here: http://www.ebay.com.au/itm/Quad-34-Complete-overhaul-upgrade-service-/130111122176?hash=item1e4b3a2b00

The main difference is, that the CD4066 switching ICs are also replaced with MAX4066CPD+ chips. They are said to offer a lower and more linear on-resistance. Stefaan and Joost rather don't think, that this makes an audible difference, but EJP thinks otherwise. I might be able to find out by comparison, once I have updated both pre amps.

One has a slightly newer board than the other. Currently I sometimes have the feeling, that the older one sounds a little better. Sometimes not. :)

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21-Aug-2015 11:22 PM

Marcel

Marcel

Posts: 21

Hi Fabian,

Thank you for your information. A few months ago I did an upgrade on my Quad 34 using the standard Dada kit to my full satisfaction. Good luck with your projects.

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23-Aug-2015 02:07 AM

Fabian

Fabian

Posts: 102

I had promised some pics of the rebuild, so here they come. Don't expect too much. I did not take the time to document all the different rebuild steps. Also, the pictures before the upgrade were taken with my iPhone and the quality is much worse.

http://www.abload.de/gallery.php?key=ucMLY38t

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23-Aug-2015 02:31 AM

Fabian

Fabian

Posts: 102

Some more comments on the upgrade itself:

The kits make the rebuild very easy. All components are clearly denoted in the upgrade manual. Read it fully before you start your revision. Also follow the advise given there and download the service manual as well, prior to doing the actual work. You don't have to understand it all, but it might e.g. be a good idea to mark the parts to exchange on the layout plan.

I even wrote the component numbers from the schema on the foil caps and on the paper flags on the resistors as a preparation. Not really necessary, but one thing less to pay attention with your head inside the amp.

Still have to complete the revision of my 34 before making a final judgement, but the amps sound really, really good and very quiet now. No noise, no humming, lots of punch. I did not hook up my scope yet (forgot where I put it and probably forgot how to use it as well :) ), but judging from the sound I don't expect to find anything strange. I did use my multimeter to check the positive and negative power rails and found that they were both aprox. 2 V down from the values in the service manual. I don't have an explanation for this yet, but again, the sound is just fine ...

If you have watched the pictures very carfully, you might have noticed, that I did not use the power caps and speaker binding posts from the kit. This might sound a bit ironic, as these are probably the most expensive components ;) . The binding posts in my kit have not been usable with banana plugs, which I still prefer. For that reason I went with Hirschmann BIL 20 AU sockets. I like them a lot. The power caps in the kit were 4700uF/63V Epcos. This is the same capacity as originally used by Quad and surely appropriate. I still orderd 6800uF/63V Panasonic caps from DADA instead, hoping for just a tad more transient power into lower loads.

If you don't have any soldering experience, the revision might still be doable as a maiden project. In this case you should make absolutely sure, that your technical equipment is complete and of very good quality. You should also check out a number of soldering and desoldering tutorials. The upgrade manual will NOT tell you which end of the soldering iron gets hot or how to use a solder sucker. However, my personal recommendation would be: Take advantage of DADA's upgrade service. This is sure more expensive than buying a kit, but it is money well spent. Personally reworking each amp took me roughly 4 hours. If you are an experienced electronics worker with a permanent workplace you will be faster, of course.
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30-Oct-2019 01:33 PM

Fabian

Fabian

Posts: 102

Some four years later and I have to confess that the upgrade was successful on one 306 only. I screwed it on the second one. Can you spot what’s wrong in this picture?

https://abload.de/img/25562_5_after_topvgsg1.jpg

Especially when compared with this one?

https://abload.de/img/20515_5_after_topgusjb.jpg

Shame on me: In the first pic, C7 (originally 47µF 63V originally, uprated to 100µF 63V with the revision kit) is mounted with  reversed polarity in the right channel (also right side of the picture). Huh …

The top of cap had started to bloat, but it did not burst, yet. Also, it still did play some music. I had been using the right channel to drive woofers only. The amps have not been in continuous use over the years, but I am surprised it still worked. My cheap LCR meter still shows a capacity of 55µF.

Unfortunately, I don’t understand the circuit well enough to know what C7 is supposed to do. Can anybody explain in simple terms what it is supposed to do (and what could happen if it fails)?

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31-Oct-2019 02:10 AM

EJP

EJP

Posts: 1669

C7 is the bootstrap capacitor. It turns the 2x560R collector load of Tr7 into more or less a current source with infinite impedance, which increases its gain enormously.
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31-Oct-2019 04:15 PM

Fabian

Fabian

Posts: 102

OK, so if C7 would have failed completely, I should have noticed the difference, I assume.

Thanks again for sharing your profound knowledge!

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