Good evening.
I have a Line Magnetic LM-508IA as my main amp. It's great, but I have a hunch that a dedicated preamp might make it even better. I am actually about to get a LM-512CA. The supposed gain of the preamp is 20db, there's not much more information, and I also couldn't get hold of a schematic. The power amplifier features a "preamp input", but it only bypasses the volume pot and not the preamp stage. My question is, whether you think it's a good idea to change this to a real bypass, and where exactly you would put the signal from the preamp. Between C103 and R110 and go directly into the 300B driver?
I attached a schematic of the LM-508IA. The first stage is half the 6SL7, cap coupled to the 6SN7, which has both triode systems working in parallel, another coupling cap and then the 300B driving the 805 as a directly coupled cathode follower.
Thanks and best regards
I have a Line Magnetic LM-508IA as my main amp. It's great, but I have a hunch that a dedicated preamp might make it even better. I am actually about to get a LM-512CA. The supposed gain of the preamp is 20db, there's not much more information, and I also couldn't get hold of a schematic. The power amplifier features a "preamp input", but it only bypasses the volume pot and not the preamp stage. My question is, whether you think it's a good idea to change this to a real bypass, and where exactly you would put the signal from the preamp. Between C103 and R110 and go directly into the 300B driver?
I attached a schematic of the LM-508IA. The first stage is half the 6SL7, cap coupled to the 6SN7, which has both triode systems working in parallel, another coupling cap and then the 300B driving the 805 as a directly coupled cathode follower.
Thanks and best regards
Attachments
There is nfb returned to the cathode of the first stage, so the input stages cannot be bypassed.
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"I have Negative Feedback set at 1, so none at all"
Look closer at the connection to R102 below switch 4, the resistance is higher than switch 2,3 or 4 but due to the connection there, you still have feedback.
I would not suggest altering a valuable amp. Amps that have been modified are of very little financial value.
Richard
Look closer at the connection to R102 below switch 4, the resistance is higher than switch 2,3 or 4 but due to the connection there, you still have feedback.
I would not suggest altering a valuable amp. Amps that have been modified are of very little financial value.
Richard
If you consider electronics a religion, a hunch is fine. If you consider it a science, it's good to have a little more than a hunch.
Whether a preamp is "dedicated" or not does not make it better or worse. Considering the price, I am not particularly impressed with the amp but there is nothing basically wrong with the first 2 stages. They are a very simple conventional design. I have no reason to believe the preamp circuit is any better or worse.
$4k for a preamp from an anonymous source with no tech info seems absurd to me, but if that doesn't bother you just buy it and see how it works out.
Your amp does have NFB in position 1, it's just very little. As far as modifying it to bypass the first 2 stages, it's doable. Just have a qualified technician perform the work.
With due respect, you don't seem to have the skills to safely work on this equipment.
I did in fact completely ignore R102. You're right, of course, there's still NFB. My bad, I'm still a little cooked from my night shifts, sorry.
I don't entirely get some of the comments though. 99 percent of people buy with their ears only, without ever understanding the amplifier designs. For many pieces of kit there are no schematics available at all. Do you avoid any amp or speaker that you can't analize beforehand categorically? I mean, it's one way, sure, but it seems weird to me.
And as for the simplicity of design - I found that many of the amps that sound best tend to be very simple. I don't think it tells you much inherently, or does it?
I don't entirely get some of the comments though. 99 percent of people buy with their ears only, without ever understanding the amplifier designs. For many pieces of kit there are no schematics available at all. Do you avoid any amp or speaker that you can't analize beforehand categorically? I mean, it's one way, sure, but it seems weird to me.
And as for the simplicity of design - I found that many of the amps that sound best tend to be very simple. I don't think it tells you much inherently, or does it?
Correct but misleading. 99+% of the general population do not buy $4k preamps, period. Of the remaining <1%, a majority buy from established firms. So yes, if I'm buying a McIntosh and I don't know anything about electronics, I assume it's well designed and I buy with my ears. Or without. I wouldn't buy a no-name amp unless it was priced at $100 or less so I could throw it away if I didn't like it.
People on this forum are NOT the general population. Most DO understand amp designs to various degrees and in fact many don't buy at all, they build their own. That's why it's called diyaudio.
Agreed. They should be as simple as possible to meet the design goals, and no simpler.