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Tri-amp using 2a3 SETs

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marcutan

Member
2006-07-23 7:12 am
Hi,

I built a carbon fiber laminated MDF loudspeaker using Raven R2-PHL 1120 and PHL 3003 drivers. The active crossover is an audio-kit crossover at 300Hz and 2.5Khz

http://www.audio-kits.com/catalog/item/4071180/3884480.htm

I build 3 channels of Sun Audio sv2a3 based SET (using a single power supply) to power one single speaker for testing. This is a simple 6sn7 cascade - 2a3 design. The sound is good and I am looking for suggestions on how to optimize my second build. Based on the initial listening, I am thinking of making the following modifications:

a) Separate power supplies for the bass channels. The current single power supplies sound insufficient during heavy music passages.

b) Using 15H/10H chokes instead of 5H. + 100uf filter capacitor for bass channel.

c) Use JJ 2a3-40W tube for bass, bias at 260v, 80mA. I feel I needed more power for bass during initial listening

d) Hashimoto Output Transformer H203-S for Highs/Mid and Lundahl LL1623 for bass. I have these.

e) Using DC heated supplies (IC regulated) for 6sn7 and 2a3 only for the bass channel. If the results is good, I might use this for the mid/highs too. Hum is currently at 4-5mV measured at woofer. 2mV at Twt and Mids. Should I use DC heated supplies for mid/highs too?

e) changing the crossover points to 400 Hz and 4KHz. Somehow, I feel that the Raven R-2 sonic signature is too different to the PHL and a higher crossover point may help the sound. Is this a good choice? Or I should drop the Raven and use a dome tweeter like Seas Millenium?

This is my first diy tube amplifier attempt and 2nd DIY loudspeaker (The first Pine wood based speaker resonates :) so I dropped it ) so any suggestions to a newcomer to this hobby are appreciated!

Thanks!
Marcus Tan
 

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EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
Almost certainly the best thing you can do is to optimize your crossover. I bought a Behringer DCX2496 purely to use as a test bed to find out what crossover is needed (I can dial in anything I like in seconds). When my new loudspeakers are built, I will use the Behringer to find out what is the optimum crossover, then build it in analogue. To do all this properly, you're going to need a loudspeaker measurement system. I use LAUD because I bought it years ago, but there are lots of more recent soundcard-based measurement systems.
 

marcutan

Member
2006-07-23 7:12 am
I struggled for 10 mins to desolder a cap from the XO PCB broad and gave up. Just purchased an Ashly crossover.

What's the best way to get good bass from 2a3? Is there any tweaks I can do to the Sun Audio schematic to get punchier tight bass?

My woofer is the PHL 3003.
 

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marcutan

Member
2006-07-23 7:12 am
For cathode bias, is it ok to increase the bypass capacitor from 100uf to 200uf? The lundahl ll1623 transformer has a 1.6k, 3k and 5k tap, I will use the 3k tap as its closest to your suggestion. Might play around with 1.6k tap to test out the difference.

Instead of using a 300b for bass, I am going for a JJ2a3-40, which is fundamentally a 300b with 2.5V filaments. However, I decided to bias it at 250v, 80mA instead of the traditional 300b bias point (350v, 75mA) as the graphs/specs shows that it is more linear at around 250v. I am using a JJ2a3 to have a more sonically balanced sound; relative to the 2a3 mid/highs.

I did consider push-pull too, but I needed to reduce costs.....building 6 channels of 2a3 SET to tri-amp has been expensive enough......I tried solid state amplification and didn't like the results (even with a tube pre amp at the front).

Thanks!
 
to get the good bass from 2A3
try with Lp of output transformer
>=22H, Ck=22uF-33uF (as higher Lp, the value of Ck decreasing...)
and lower value of C at Vb side at OT of about 16uF to 22uF...
-3db around 8Hz and flat, no peek
on the other side of BW find the Cdyn fot 2A3 in the circuit...
for the -3db at 131KHz it is about 16.5Kohm og Req of driver/input tube.
for instance 12ay7 in parallel...
or maybe Loftin-White config.
just one thing.
with a larger value of Lp You could go for bigger V swing then -Ug
with no distorsion
but more Lp brings the total capacitance larger which causes roll-off
highs...
RL of 2.5K fot the load is for up to 10% distorsion...
RL of 3.12K is better.
settings are
Vb=272V
-Ug=50V
Ia=55mA
still fit into 15W disipation...
power supply, L-load, mercury rectifier...
and if You can find
2A3 with single anode
You ill have top amp
cheers
 

marcutan

Member
2006-07-23 7:12 am
What's Vb? I couldn't grasp what's Cdyn either. The bypass capacitance (Ck) of ~20-30uf seems low but I would test out your suggestion.

The lundahl ll1623 has a Lp of 30H and static sec DC resistance of 0.8ohm. That fits your criteria. Its specification suggests using the 1.6Kohm primary for better bass headroom. But I am thinking of 3k as what you suggested.

The output transformer I have is gapped for 90mA. Based on these considerations and suggestion: I will design an operating point of 275V, 80mA and -50v.

Thanks! Appreciate your suggestion.
 

marcutan

Member
2006-07-23 7:12 am
Hi,

Thanks. 530v is too high for the power supply parts that I have now. It will be great to try out the DRD design some day though.

I am thinking about a redesign of the speaker too. I have a 42L box for the PHL3003 woofer. I might change to 24L for a faster but lighter bass.
 
EC8010 said:
Almost certainly the best thing you can do is to optimize your crossover. I bought a Behringer DCX2496 purely to use as a test bed to find out what crossover is needed (I can dial in anything I like in seconds). When my new loudspeakers are built, I will use the Behringer to find out what is the optimum crossover, then build it in analogue. To do all this properly, you're going to need a loudspeaker measurement system. I use LAUD because I bought it years ago, but there are lots of more recent soundcard-based measurement systems.


At last!!! A man who uses a Behringer Ultra-drive properly. My faith in this forum is restored and, yes, I wholeheartedly agree. Excellent way to determine crossover points and slopes. 1 word of warning though. The 2496 was designed as a rack controller for pro audio. Used as it was designed, the 48dB/Octave crossover slope is useful in room but I would seriously go no higher than 24dB/ Linkwitz Reilly for HiFi. I wonder if EC8010 agrees with me?
 
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