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Old 22nd February 2009, 03:10 PM   #1
kendt is offline kendt  Canada
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Default need amp suggestions please.

I have a pair of speakers that use the fsr125 in an mtm setup. I am going to add side firing sdx7's for bass. I think they will sound awesome when they're done. The problem being the low sensitvity.

I need to build an amp to drive them and so far i'm not having much luck. I would really appreciate some direction.

Here are the requirements.

I prefer single end over pp
Must have enough power to drive speakers around 85db's to reasonable levels. Doesn't need to blow the roof off though
I love the sound of 300b's, 2a3's 45's etc. So I would like something that has a similar type of sound.
Must be reasonably simple to build. I have built a couple in the past but i'm no expert.
Must be able to sound good without a fortune in exotic caps and transformers.
Prefer an integrated type amp but could build a preamp if I had to.

Thanks Up front.
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Old 22nd February 2009, 04:32 PM   #2
kevinkr is offline kevinkr  United States
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I would recommend first picking up Morgan Jone's "Valve Amplifiers" 3rd edition, and "Building Valve Amplifiers" if you do not already have them.

The big problem I see with your wish list is SE in conjunction with an efficiency of 85dBspl. You will either need a tiny room, or be satisfied with relatively low spls. An 8W 300B SE amplifier will get you to less than 93dBSpl at a meter from the speaker, and while loud it will be with lousy linearity - and in fact may not be loud enough. In my 200sq ft listening room 8Wrms with 100dB speaker system is adequate, and not more in terms of achievable clean spls..

You might consider something along the lines of the old Borbely 6C33 design with 15Wrms output that appeared in Glass Audio many years ago or Romy The Cat's Melquiades here: http://www.goodsoundclub.com/Melquiades.aspx The output transformer in such an amplifier would have to handle more than 200mA of plate current - a tall order, but primary Z is a low 600 - 1K making it a little easier, but this might still not be enough power. Romy's recommendation of 100dB efficiency at this power level is in line with my own experience.

A PP DHT amplifier with around 30Wrms output would be more suitable and iron is more readily obtained. I did a design for VTV about 12yrs ago using 300B in 0fdbk PP and this design has been replicated successfully dozens of times over the years. Link here: http://www.kta-hifi.net/projects/amp...mp/300bpp.html (Note that I no longer provide the information to build this design for free on my site, need to pay my web host and if that is a consideration there are plenty of free designs on the web. Or get an old copy of VTV )

Lynn Olson's Karna is another more complex possibility with great performance as well. Here is a link to the Karna: http://www.nutshellhifi.com/triode2.html

Good luck! I started with speakers in this efficiency range and drove them with 70W home brew cathode coupled fdbk PP amplifiers and I found that more than adequate.
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Old 22nd February 2009, 05:12 PM   #3
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Kevin has already alluded to the considerable difficulties, both technical and financial, in achieving your objective.

You can get plenty of power and very good sound out of PP circuitry, if you incorporate the 12AT7 into the small signal circuitry. The non-linearity of the 'T7 works to your advantage in PP designs. The NET harmonic distortion spectrum is an ear pleasing "waterfall" of 2nd > 3rd > 4th > 5th.

Apply Paul Joppa's 102 dB. rule to your 85 dB. efficient speakers. The peak power requirements are fairly formidible. No SE design that's remotely affordable delivers the power needed. A 300B SET amp that yields 8 WPC requires speakers no less than 93 dB. efficient to satisfy Joppa's Rule.

BTW, don't forget to factor speaker cross over elements into the equation.
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Old 22nd February 2009, 05:47 PM   #4
kendt is offline kendt  Canada
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Thanks for the links I had not seen these yet and will look them over.

I like the sound of 300bs a lot but i realize they won't work in my situation. I only hope to get as close to them as I can with other tubes. I have looked at the amps built around 6550's and they seem to get fairly good remarks about they're sound quality. I thought of trying one of them but doubling the output tubes for more power. I am not sure though if I would be asking for trouble or if it would be fairly straight forward.

I am willing to buy decent transformers but cant afford the high end ones.
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Old 22nd February 2009, 06:27 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by kendt
Thanks for the links I had not seen these yet and will look them over.

I like the sound of 300bs a lot but i realize they won't work in my situation. I only hope to get as close to them as I can with other tubes. I have looked at the amps built around 6550's and they seem to get fairly good remarks about they're sound quality. I thought of trying one of them but doubling the output tubes for more power. I am not sure though if I would be asking for trouble or if it would be fairly straight forward.

I am willing to buy decent transformers but cant afford the high end ones.
One of Edcor's better PP OPT models and a pair of 6550/T88 in UL connection will get you nicely into the 50Wrms + range per channel - this will be more than enough power for your needs. Here: http://www.edcorusa.com/products/tra...-4_8-4_2k.html
OR
http://www.edcorusa.com/products/tra...0-ms-4_2k.html which would allow you to experiment with symmetrical cathode feedback in the output stage. (Note this does not work well with every transformer out there, but when it works well, it works very well.)

Lots of driver stage choices lurking out there, I tend to favor differential topologies (typically a pair of direct coupled LTPs) with no more than 20dB of global feedback applied.
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Old 22nd February 2009, 06:34 PM   #6
kevinkr is offline kevinkr  United States
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Note on the 300B, there is no reason why you can't use them in PP and get 30Wrms out of a pair using the Edcore transformers I mentioned above. Fixed bias and 400V on the plates - the tubes are kind of pricey as only good ones survive at these voltages.

I use JJ 300B, and note that you can also use the JJ 2A3-40 with 2.5VAC heating and not have any hum issues in a PP amplifier. (In fact I never use dc on 2.5V filament power tubes as they seem quiet enough - such is usually not the case even in PP with most 300B)
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Old 22nd February 2009, 07:23 PM   #7
kendt is offline kendt  Canada
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To be honest I have only heard a couple pp amps and they weren't really my cup o tea. I know they make way more power but I'd prefer to go the se route if at all possible.
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Old 22nd February 2009, 07:52 PM   #8
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Quote:
Originally posted by kendt
To be honest I have only heard a couple pp amps and they weren't really my cup o tea. I know they make way more power but I'd prefer to go the se route if at all possible.
What sort of PP? Class A triode or some typical pentode/UL design?
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Old 22nd February 2009, 09:48 PM   #9
kevinkr is offline kevinkr  United States
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Seriously then you need to consider a speaker system design that plays nicely with the typical 300B SE amplifier, not the other way round. (That is in fact what I did.)

A high powered SE amplifier is going to cost you a lot of money, and you indicated that is a problem. My last SE amplifier built 5 yrs ago cost over $3K to build and that was with a very large number of parts on hand - replicating a similar design of a similar quality and the much higher power you need will cost a lot more.
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Old 22nd February 2009, 10:34 PM   #10
kendt is offline kendt  Canada
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Yes the more I look into it the more I'm thinking the speakers have to go Which really sucks cause they already sound great and they're not even finished yet.

I may have to look into some type of front horn that would capture the qualities of the fr125 drivers but increase the efficiency somewhat.
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