M2 has 15dB gain already. It’s good to aim for 30dB total so adjust preamp to have 15dB of gain or thereabouts.
Change R11 to 2k2 and you should have about 15dB gain in Preamp
Thank you, X! As always, good and to the point advice!
About the ALPHA; I've already received the M2 boards and autoformers (from Tea-bag). Getting matched pairs of Toshiba JFETs may be a problem, but I can always opt for one of the M2x daughter card circuits with easier to obtain components.
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Twocents (Jerry) may be building the Alpha 20. If he does, we can compare it to the M2. And if I like what I hear in the Alpha 20, then maybe I'll build one too. I think there is still a lot of soldering laying ahead for me, but I'm enjoying it.
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Yip, that is the plan and I have the perfect preamp (once it is done) for it!
Twocents (Jerry) may be building the Alpha 20. If he does, we can compare it to the M2. And if I like what I hear in the Alpha 20, then maybe I'll build one too. I think there is still a lot of soldering laying ahead for me, but I'm enjoying it.
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Will be good to hear your side-by-side comparisons.
Ok, great. So one should leave R10 at 10k. I assume that rather increasing R11 to 2k2 will apply for use with an ACA (14dB) amp too?
We will most certainly let you know our impressions. You've been a great help in this audio adventure, X.
But before that happens, I'll have to build bigger speakers. The 0.53K's (dual Faital) are unbelievably good with my ACA since I've upgraded interconnects and speaker cables. However, I doubt that they will do the M2 and Aksa Lender preamp justice. Any speaker suggestions?
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The M2 and Alpha 20 are similar low wattage 20-25w amps and really benefit from speakers that are at least 89dB sensitive. Dual 3FE25-16’s in parallel are 97dB sensitive at 8ohms and can handle 30w I believe. They are actually very well matched for the M2. You can look around for 89dB+ two way speaker systems. They tend to be either pro audio 12in woofers and compression driver tweeters. Kits for this type of speaker can be bought from DIY Sound Group. Names like Tempest etc. all very cost effective but you live with the sound of a CD. If you like CD’s that’s fine.
Or there are some with dome tweeters. But tough to get 89dB sensitivity from standard HiFi woofer as -5dB baffle step means woofer needs to be 94dB sensitive. They just don’t exist. So maybe (dual 8in woofer rated st 88dB in parallel for 94dB) and and a full range top 3FE25? That gets you the powerful dynamics of dual 8in woofers and overall system around 89dB. So sort of like my RS225 FAST but with dual 8in woofers and a single 3FE25-8 (or W5-2143) for the top. A new crossover would have to be designed though. It would sound really good and be transient oerfect 1st order crossover.
Another very high sensitivity speaker is the Elsinore as it has lots of woofers. It’s kind of pricey to make though as parts count is high, but I have heard good things about it.
Or there are some with dome tweeters. But tough to get 89dB sensitivity from standard HiFi woofer as -5dB baffle step means woofer needs to be 94dB sensitive. They just don’t exist. So maybe (dual 8in woofer rated st 88dB in parallel for 94dB) and and a full range top 3FE25? That gets you the powerful dynamics of dual 8in woofers and overall system around 89dB. So sort of like my RS225 FAST but with dual 8in woofers and a single 3FE25-8 (or W5-2143) for the top. A new crossover would have to be designed though. It would sound really good and be transient oerfect 1st order crossover.
Another very high sensitivity speaker is the Elsinore as it has lots of woofers. It’s kind of pricey to make though as parts count is high, but I have heard good things about it.
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Thank you for the suggestions, X. I'll do a bit of research on the mentioned options. I understand what you're saying about my Karlsonators being good enough, and I'll try them out, but I have a feeling that I need to go bigger with the M2 and ALPHA 20.
With the improvements on my ACA, the K's are already driven to their max on low frequencies. Let me explain... I dumped the baffle steps in favour of the built-in equalizer in Winamp (yes I know there are better players, but my old laptop can't handle them when EQ is used). In EQ I've cut hi freq and boosted the lows a bit. The result is a really great sound, but those small drivers are having a hard time to make big bass. At low volume they sound super (for their size).
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Secondary outputs
X, for adding a secondary output you recommended to increase the bias slightly by changing the following resistors: R14 – from 10R0 to 6R8, and R15 – from 47R5 to 33R0. Then I assume one simply split the output signal to two sets of RCAs? Will this work fine for bi-amping purposes?
I don’t understand the technicalities and I don’t know how it is done on other preamps, but a secondary output was implemented slightly differently on the B1, by duplicating the output resistor (suggested by the late AndrewT I think and confirmed by Mr. Pass). See the post here:
B1 Buffer Preamp
X, for adding a secondary output you recommended to increase the bias slightly by changing the following resistors: R14 – from 10R0 to 6R8, and R15 – from 47R5 to 33R0. Then I assume one simply split the output signal to two sets of RCAs? Will this work fine for bi-amping purposes?
I don’t understand the technicalities and I don’t know how it is done on other preamps, but a secondary output was implemented slightly differently on the B1, by duplicating the output resistor (suggested by the late AndrewT I think and confirmed by Mr. Pass). See the post here:
B1 Buffer Preamp
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Congratulations JW, beautiful work!
The distinction of the AKSA/Lender preamp is the spatial information. When you listen to clapping from a rock concert, close your eyes and perceive the depth and width of the sound stage. This is a tube trick, and one of the reasons we love tube amps. It makes the music so intense, you can resolve better because you can focus on the instrument space.
The 2SA1837 from Toshiba, now coming out of production, is one of the best transistors ever made. It is a beautiful device, rated to 70MHz, Ic 2A with surge to 4A, and very low parasitics and high hfe. One of the best drivers ever made; complement is 2SC4793.
HD
The distinction of the AKSA/Lender preamp is the spatial information. When you listen to clapping from a rock concert, close your eyes and perceive the depth and width of the sound stage. This is a tube trick, and one of the reasons we love tube amps. It makes the music so intense, you can resolve better because you can focus on the instrument space.
The 2SA1837 from Toshiba, now coming out of production, is one of the best transistors ever made. It is a beautiful device, rated to 70MHz, Ic 2A with surge to 4A, and very low parasitics and high hfe. One of the best drivers ever made; complement is 2SC4793.
HD
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Great stuff, cannot wait to finish mine and experience this.
Hugh, if using the 2SA1837 in the Lender, would you recommend using the 2SC4793 also, iso the Fairchild KSC1845? Would that make a difference?
I’m not recalling where I made that recommendation (can you show me link)? Here is how to change it to be a HPA:
If driving two loads - increasing bias helps provide enough current. Although if loads are high impedance amp inputs (>10kohm ea) you may be fine with stock settings.
I am not interested in a HPA at this early stage, only a decent preamp. I am thinking of WAW experiments later, which will involve bi-amping and possible PLLXOs, that is why a second output on the preamp would be nice. That recommendation was in an email regarding a second output - but I think it is better to discuss here. You said:
"That’s fine as long as the combined impedance is not too low. You can also boost bias current to handle more output. The source resistor on the CCS DN2540 controls that. So change R15 to 33R and R14 to 6.8R to boost bias and drive lower impedance loads. Stock setup can drive 4k7 load no problem."
Thanks for all the help and guidance so far!
No 2c, I would not use C4793 for the VAS. It will work well, but it's only running very low current, only 1.5mA, and the larger Toshiba is rated up to 2000mA. 1.5mA is too low and beta will be low at this current. The much smaller C1845 would be superior here; the smaller transistor has Cout of 1.6pF, typical hfe is 600, and it's very low noise, around 25mV at 1mA. The C4793 is comparatively slower with Cout 20pF, typical hfe of only 200, but at only 1.5mA the speed is way down, maybe around 20MHz. But it would work. In this role the dissipation of the C1845 is around 30mW; the C4793 will dissipate up to 20W heatsinked so this 30mW use is too low.
HD
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