UCD180 questions

[pa]

Hi,
It seems that ucd180 are out of stock at the moment, are they coming back with a new version or some modifications ?

[Jan-Peter]

Pa,

Next year around 10th of January there are again available.

Regards,

Jan-Peter

[maxlorenz]

I'm feeding my UCD amp through a NAD receiver as preamp (no hum/buzz whatsoever). I have a channel imbalance: one UCD seem to be outputing 2 to 3dB more than his sibling
I noticed this because soloist singers migrate to one side of the room without being invited
I didn't have time to check other preamp/amp combination.

Could it be possible that one UCD has lower gain?

Appart that the amp sounds terrific! Very natural sounding, transparent and effortless. Great bass output for US$15 worth of PS caps

Unfortunately, I still have a buzz when connecting it to my beloved TVC (transformer based volume control). I have a hunch that I can convert it to a balanced pre (UCD's input driver doesn't need perfectly symetrical/opposite signal, wright ? )

I'm starting to dream about a UCD400 stereo amp with HiEnd parts: I only have to get the money for it

Regards
Mauricio

[eboissier]

Hi everybody (this is my first post here)....

Well, I've just finish to read all that looonng thread (dont remember everything in it but I've done it ...)

I plan ton build a little amp with two UCD180 and I dont have a preamp...

Is it possible to just put a log potentiometer (or attenuator) on the input signal ? no impedance problem ? input level sensibility is OK ?

Thanks,
Eric

[ewildgoose]

Yep. Cheap maplins attenuator should work fine and at very high quality even.

The basic rule is that the cheap variable attenuators have poor channel balance and it's worse on the log versions. Some people use a linear pot and what they call a law faking fixed resistor in parallel.

With a balanced input just put two attenuators on each channel (one for hot, one for cold). You can take the maplins cheap attenuators apart and join them back together as a really large multichannel version...!

The "best" attenuators are expensive and are either the Dact stepped things, or a more custom service of the same stuff from www.goldpt.com. Basically though if you have patience these are just high quality Elma switches and you could make your own if you can solder lots of resistors into a very small space...

There is no magic here though. Just try a cheap pot first and see how you go. Move up from there. It's unlikely that you need extra gain in your preamp, so bar hum problems you should be very happy with a passive preamp.

Oh, there is one other design of passive preamp which is popular and that's the transformer designs. See Steve and Billinghams and another name that I forget now, for these pretty large transformers with loads of secondaries, one for each volume that you might want to step down to. Generally considered to be "slightly better" than the attenuator option. Also expensive though

[tiki]

I used a double log pot (10k) per channel, like ewildgoose suggested. Connected (right) ends together to screen of the cable (possibly put them together also, if 2 channels are used, not perfect, I know), wipers to the inputs of the UcD, left ends to the XLR-input connectors. Input connector wiring to the pot and wiring to UcD inputs use separate shielded cables. Shield is connected to the XLR, pin1, which is connected to the chassis too.

The poor balance of double log pots degrade the CMRR of the amps a bit, but until now I can live with that (could exhibit more hum, if picked up anywhere).
This simple solution exhibits a possibly variable load to the preamp, dependent on the UcD input impedance. It would be more elegant to put buffers between XLR and pots, pots and UcD respectively, I was to lazy for that.

I'm still looking for a PGA-solution for use behind the Behringer DCX.

Timo

[rha61]

Originally posted by maxlorenz
ALAIN:

<<i've replaced my Elna main caps with big EPCOS Sikorel 105 15000/63 and it's an improvement (the best caps i've tried )>>

While everybody is happy with metal boxes I found your last post to be most interesting (I almost buy Elna Cerafines). I've search and I only found Sikorel caps for automotive applications. Are they these? Please tell us where to buy them (and also in the Americas) and what diffrences did you find between these and other caps.

Merci beaucoup...

Mauricio

Hi Mauricio

I've had some problems with one of my modules because of too much experimentations ( i've sent it twice to Jan-Peter ! )

With the Sikorel , i was expecting big bass because of their very big size but it's in the midrange-treble area that something happens , the bass are normal
the Sikorel capacitors increase the natural sounding faculty of the UCD because of a very fluid and transparent midrange-treble , they sound like a big film capacitor
I know that unfortunately they are difficult to find , i've got mine by phoning directly to the french distributor

alain

[maxlorenz]

Bass is already very good with the UCD amp! I've got 10000uF*2 + 4700uF *2 commom type caps.
I was lucky and found that rs-export.com has all kind of big cans and I asked for BHC Aerovox 22000*2/63V. This will be "only" my secondary active speaker system. I'll will ask for Sikorel for the next UCD400 project

I think now I have to find a good, cheap balanced preamp to buy or build. Any suggestions welcome.
Maybe I'll try some "pro" pre with tube buffer for fun.

Sorry to hear about your troubles with the UCD. I hope they will last long if one treates them well. I mean Mosfets running very cool and with overvoltage protection must endure, wright?

Au revoir.
Mauricio

[ewildgoose]

You can't get much cheaper than the passive variable resistor preamp as described above... Get a 4 gang one to make it a balanced version (or do the dodgy hookup that I am using which makes a balanced channel using one attenuator per channel... (According to Bruno it's not a good design, but it's "working ok here"...)

I doubt you will easily outperform even a cheap attenuator with an active pre. My £2 maplins job seems quite comparable to my old Meridian 502 pre amp...

[mac]

I have a question regarding the /ON operation of the UcD modules.

I have three UcD modules per chassis and wondering if a common single pole switch can be used to turn off/on all three modules. Since each of my modules has its own power supply and there is no common chassis ground can I use the secondary center tap of any of my three modules?

My plan is to keep the V+/- active all the time and use the /ON pin to turn the amps off and on.

Thanks, mac.