Rozenblit 300B SEOTL 1.5W

What people are working on at the moment
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#1 Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

I've just about finished testing a Rozenblit 300B single ended OTL set of monoblocks, I'm building for one of our Ant's clients.

Ray asked me if I would put something up about it, so here are a few pics of insides:

Amp 1:
Image

Amp 2:
Image

25Hz filament supply:
Image
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#2 Re: Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

The curse of the OTL struck when a bad Sophia Electric mesh plate arced over and took out the 36V zener that was part of the mod introduced, that protects the CCS that biases the output stage. God was I cursing! What the hell is it with me and OTLs? :x

A bit of fault finding revealed that the zener had failed short circuit as a result of the arc.

I replaced the CCS chip as well as the zener as a precautionary measure.

There's a whole thread on the Transcendent forum about the perils of using Sophias in this amp.

Anyway I had no trouble with the four new EH gold grid 300Bs that Ant's client sent me the other day, and after fixing the faulty monoblock, I was able to test each one individually using them. Everything worked perfectly.
Last edited by Cressy Snr on Thu Feb 23, 2017 10:51 pm, edited 2 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#3 Re: Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

With the caveat that I was lisening in mono to mono recordings, the sound has a hyper-real quality about it. Great on girl/guitar stuff, but on my speakers it lacks the authority that the Barbarik pentode amp has. When the going gets loud, you hear loud, probably because my OmniMets are not efficient enough. The thing is likely to be clipping most of the time at normal levels on my speakers.

There is that SE second harmonic love about the sound, that I don't experience with the PP pentode amp, and frankly I can do without it, but YMMV and all that :) .

Now there's no doubt at all in my mind that with the right speakers it will be a phenomenal proposition, but TBH I much prefer the PP sound of Dave's "Son of Beast" OTL on my speakers.

It's not for me, but If you like the SET sound and have 100dB efficient horns as our Ant's client has, or big baffles, you will without a doubt, absolutely love the sound this amp system gives and I'm certain he will adore what it does. It does SET like nothing else I've ever heard. An extremely niche product, but within that niche, it would take some doing to better it.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#4 Re: Rozenblit 300B SEOTL 1.5W

Post by IslandPink »

Interesting and not unexpected results there Steve. It would be very interesting to run the upper half of my horn system from an OTL at some stage, Nick has discussed it with me once or twice. Just a shame they're so inefficient , it's still a biggish amp in terms of size & power draw , for 1.5w.
"Once you find out ... the Circumstances ; then you can go out"
User avatar
Ali Tait
Eternally single
Posts: 4373
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#5 Re: Rozenblit 300B SEOTL 1.5W

Post by Ali Tait »

Built a Mini Beast recently Mark, using it for the top end on my OB's. Good amp.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#6 Re: Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

Ant's client is not collecting for two weeks, so if anyone has four decent 300Bs of the same make (no meshies FFS) and fancies a listen to the amps in stereo, they are welcome to come round. Apart from anything else' it'll give a chance for a run in and soak test of the things before he collects.

Post here or shoot me a PM if interested.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Ray P
No idea why I do this anymore
Posts: 6294
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

#7 Re: Rozenblit 300B SEOTL 1.5W

Post by Ray P »

Thanks for posting this Steve.

I use my pair with Lowther BL horns and they don't run out of steam in my domestic context. They are very special IMO.

My build uses DC filament supplies for the 300Bs (using Andrew's DHT supplies). I have never used the 25Hz filament supplies, going straight from 50Hz AC to DC. Wolfgang, the guy over on the Transcendent forum who I worked with on the DC build previously owned the 25Hz version but rates the DC version as superior in all respects so I suggest an upgrade would be worth considering for Ant's customer at some point. Rozenblit did change his kit to use DC filament supplies but compromised (too much IMO) on the filament supplies by basing them, I believe, on simple LM317 regulators.

If anyone has the required four 300Bs and lives near Steve I recommend taking up his offer.

As Ali has pointed out, the Transcendent Minibeast SE-OTL is a great amplifier too, you'll get most of the performance of the 300B SE-OTL and a bit more output (4watts) for a lot less outlay.

http://www.transcendentsound.com/Mini_Beast.html

Ray
Sorry, I couldn't resist!
User avatar
Toppsy
Shed dweller
Posts: 2400
Joined: Mon Jul 23, 2007 12:10 pm
Location: red rose country

#8 Re: Rozenblit 300B SEOTL 1.5W

Post by Toppsy »

Cressy Snr wrote: Thu Feb 23, 2017 10:29 pm With the caveat that I was lisening in mono to mono recordings, the sound has a hyper-real quality about it. Great on girl/guitar stuff, but on my speakers it lacks the authority that the Barbarik pentode amp has. When the going gets loud, you hear loud, probably because my OmniMets are not efficient enough. The thing is likely to be clipping most of the time at normal levels on my speakers.
Not surprised you are finding the OTL 300b amps not able to drive your Omni-mets. Though your main drivers have a published sensitivity of 96dB/1W/1m this is a direct on-axis measurement at 1m from the driver. Your drivers are 90-deg off-axis to the listener and so the sensitivity will be much reduced to probably around 90dB at the listening position. Just one of the down sides to upward firing drivers and nothing one can do to redress this. If you check out the manufacturers published FR for the driver you can see that the 45-deg off axis FR plot is up to -10dB down in places on the on-axis plot. Little wonder the little 1.5W amp is running out of steam.

Image
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#9 Re: Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

Aye,
All the above was factored into the OmniMet design at the planning stage, once I had decided exactly what it was I wanted. The forward firing tweeter, in addition to providing better localisation cues, also picks up the overall system efficiency above the upper mids. A further pickup in overall efficiency is provided by the reflected sound, off the closely coupled back wall and (to a lesser extent) the ceiling.

The simple cap filter allows significant tweeter output below its theoretical XO frequency, as it is only rolling out below XO at 6dB/octave, so the net effect of this is that as the bass/mid rolls off at the rate shown on the off axis plot (a bit steeper actually as we are at around 70 degrees off the main axis when in the listening seat) the tweeter rolls in at near as dammit, the same rate. It works to give a beautifully even in-room response, with very low coloration, as both my own ears and the third octave spectrum analyser plot tell me.

The bass/mid and the tweeter were carefully chosen to (in theory) work well together in this semi-omni configuration. The results tend to bear out the theory, and due of the very high quality HF these ring radiators give, we have a double bonus of a lovely treble tone.

EDIT here's the relevant explanation from my OmniMet thread:
Cressy Snr wrote: Sat Jan 28, 2017 10:21 am
Paul Barker wrote: As long as the voicing is right in an amp (can voice a violin) I forget most other issues. No amp is perfect.


So...with the OmniMets I just dropped the full range driver idea completely and used a pro, bass/mid driver to keep the efficiency up and combined it with a good quality domestic hi-fi tweeter, in order to keep the treble from stripping your tooth enamel.
The pro driver response curve was deliberately chosen for a steady, relatively peak free top end, off-axis roll off.
The 6PR110 pro driver has a roughly 6dB/octave fall off at 45 degrees off-axis, up to around 10Khz, then a rapid drop after that.

The tweeter is brought in on a simple, first-order cap filter which mirrors the 6dB/octave main driver, off-axis roll down. The relative efficiencies of the two drivers, when the main is listened to a long way off axis and energy is dispersed in all directions, as in this omni design, means that no padding resistor to the tweeter is needed, so things are about as simple as it gets, in terms of crossovers.

These are actually more efficient in-room, than the big ones Dave now has, when you take into account the overall room boost this design (and similar ones such as the NVA Cube) gives.

Still, all that being said, they are still not efficient enough for a 1.5W amp. Nevertheless, they can still show the capabilities of the 300B OTL.
Last edited by Cressy Snr on Sun Feb 26, 2017 8:20 pm, edited 3 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Andrew
Eternally single
Posts: 4206
Joined: Thu May 24, 2007 2:18 pm

#10 Re: Rozenblit 300B SEOTL 1.5W

Post by Andrew »

I'm still hoping to hear Ray's at some point.
Analogue, the lost world that lies between 0 and 1.
User avatar
Ray P
No idea why I do this anymore
Posts: 6294
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

#11 Re: Rozenblit 300B SEOTL 1.5W

Post by Ray P »

Andrew wrote: Sun Feb 26, 2017 7:19 pm I'm still hoping to hear Ray's at some point.
I would bring them to Owston but they'll under achieve in the large room with sub-optimal speaker efficiency so not sure it would be worth the effort?

Ray
Sorry, I couldn't resist!
Andrew
Eternally single
Posts: 4206
Joined: Thu May 24, 2007 2:18 pm

#12 Re: Rozenblit 300B SEOTL 1.5W

Post by Andrew »

Thanks for the offer, but I agree it would be better to listen to them to in a more realistic environment.

Andrew
Analogue, the lost world that lies between 0 and 1.
User avatar
Ali Tait
Eternally single
Posts: 4373
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#13 Re: Rozenblit 300B SEOTL 1.5W

Post by Ali Tait »

Andrew, I could bring round the Mini Beast if you wish?
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#14 Re: Rozenblit 300B SEOTL 1.5W

Post by Cressy Snr »

Ant's customer sent me another set of EH300B, so I have set up the amps in my system in stereo.

Image

They have been running for nearly an hour and the blue glow inside some of the valves is starting to disappear as the amps warm through. In fact, since the pic was taken, the glow is just about gone. There is no hum whatsoever, under no-signal conditions.
I'm watching the things like a hawk at the moment for, as Morgan Jones puts it, "any incipient signs of self-immolation"
So far so good.

As to the sound now in proper stereo .....jeez, they're good!


Obviously they don't have the heft or dynamics of my pp monoblocks, but.....well they could have me liking the SET sound. Clarity and musicality oozes out of them, and they make the OmniMets sing.
I was definitely being somewhat unfair to the amps when I only had mono on one amp and speaker. As a stereo pair, they are actually a lovely set of amplifiers, they really are. Ant's customer is going to be over the moon with them. :mrgreen:

He's coming to my place to collect, on Sunday 19th March, so anyone who wishes to have a listen, has until the day before then to get themselves round here. If you want to hear them, DO IT! Take up the offer like your uncle Ray advised a few posts ago, there is now no need to bring your own tubes, and you'd be in for a musical and sonic treat.

What's not to like?
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Ray P
No idea why I do this anymore
Posts: 6294
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

#15 Re: Rozenblit 300B SEOTL 1.5W

Post by Ray P »

See, I told you so! :thumbup:
Sorry, I couldn't resist!
Post Reply