There is a Mesa Boogie Strategy 400 for sale in my area and I'm curious how it might fair as a hi-fi amplifier. From reading the lit on the amp they claim that, "the Strategy excels in
playback systems-especially home stereo". Looking at the schematics Here, Here and Here it looks like it could be the basis of a great stereo amp. Correct me if I am wrong but it looks to me like it would just need the front end and preamp sections tweaked for HIFI duty. Does anyone have an idea on the quality of the output transformers? From the pics here they look pretty beefy.
If this was an amp you were modifying what would you do?
playback systems-especially home stereo". Looking at the schematics Here, Here and Here it looks like it could be the basis of a great stereo amp. Correct me if I am wrong but it looks to me like it would just need the front end and preamp sections tweaked for HIFI duty. Does anyone have an idea on the quality of the output transformers? From the pics here they look pretty beefy.
If this was an amp you were modifying what would you do?
Although you cannot see frequency response from the size of a transformer for a given power output. You can make some assumptions based on size, I don't think that I will have trouble on the low end. The only way to know any transformers performance is to test and then listen to them yourself in the first place. Something I cannot due without buying or at least demoing the amplifier.Here is a link to a guy who was using one on his maggies. I also found a link to output transformers but I don't see any specs.
You are absolutely right. I replaced my diy-amps 5k Hammond output transformers to 5k Lundahls, they are small in size comparison , but much better perfomance above 10kHz, at least without feedback. Sound is also more refined.
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OK so how do you propose I do something about that? I have no way of testing the amp unless I own it. I don't know how many poorly made transformers were manufactured in the late 80s to very early 90s. I can only go off the data I have available which is the circuit design and anecdotal information. Are you focusing on the transformer due to the fact that you think it is the weak link in the circuit? If so I could always replace them with something better at some point. Or are you saying that the potential of this amp for stereo use is so dependent on how the transformers perform that it may not be worth messing with. I'm really looking for input on the quality of the circuit design and possible improvements but I don't waste my time if this isn't a good place to start.
OK. Scrap the preamp. Scrap the graphic equaliser. Scrap the OPT. Convert the output from pentode mode to either UL or triode mode - either will require the appropriate OPT. Add negative feedback - may require more front end gain. Change V4 (driver) to something more beefy then 12AX7 - and change resistor values to suit.
You end up with virtually a new amplifier which just happens to use some components from the current circuit. May be better to build from scratch.
You end up with virtually a new amplifier which just happens to use some components from the current circuit. May be better to build from scratch.
Yup, and add another channel. Most listen in stereo these days.
jeff
This is a stereo amplifier.
Ok so it looks like I posted the schematics for the bass amp, not the stereo. Here is a page with the proper schematic. Or at least it looks a little closer.
This doesn't have the EQ or preamp in it and seems to make more sense from the pics I have seen of the amp. DF96 lets say I want to keep the amp a PPP for the output stage. The front end and driver look pretty standard to me as well as the FB setup. Would you still scrap it? The only thing I think I would do is convert the 12ax7s to LED bias or add a CCS.
Assuming I could get the amp for a reasonable price, do you still think it's a lost cause? I'm not sure I could build an amp like that for $400-700.
It's a pretty standard circuit, though care has to be taken to keep things stable because of the number of stages enclosed in the feedback loop. There might be some interesting tweaks possible, starting with regulation of the screens. In any case, what you're paying for, really, is the iron and the chassis. Compared to those costs, the input circuit is trivial.
The sensitivity is very high (0.2V), and that can be problematic in a home stereo situation, where 2.0V is more common. This may be an indication that the feedback is low, which makes one suspicious about the transformer quality (usually the limiting factor in how much feedback can be applied without instability). One interesting possibility would be to experiment with local feedback from the output stage plates, which can reduce issues with the output transformer and perhaps stabilize things.
The user manual is scary- their description of feedback is terribly wrong.
Going price for used units seems to be $500-600 or so, which won't be too painful if it doesn't work out. If you want to do a lot of experimentation, it might be a good test bed. If you go that route, first thing you'll want to do is measure the output transformers' balance, primary inductance, and leakage inductance.
The sensitivity is very high (0.2V), and that can be problematic in a home stereo situation, where 2.0V is more common. This may be an indication that the feedback is low, which makes one suspicious about the transformer quality (usually the limiting factor in how much feedback can be applied without instability). One interesting possibility would be to experiment with local feedback from the output stage plates, which can reduce issues with the output transformer and perhaps stabilize things.
The user manual is scary- their description of feedback is terribly wrong.
Going price for used units seems to be $500-600 or so, which won't be too painful if it doesn't work out. If you want to do a lot of experimentation, it might be a good test bed. If you go that route, first thing you'll want to do is measure the output transformers' balance, primary inductance, and leakage inductance.
What are the two small-signal valves? Please don't tell me 12AX7 with such small anode resistors!
The first stage tail (CCS?) will have very little voltage to work with so there is scope for non-linearity. This may appear as common-mode distortion due to the way feedback is applied.
The driver stage is unbalanced both at AC and DC.
The first stage tail (CCS?) will have very little voltage to work with so there is scope for non-linearity. This may appear as common-mode distortion due to the way feedback is applied.
The driver stage is unbalanced both at AC and DC.
its a decent amp stock for home stereo - coupling caps IIRC are Orange Drop - not fired mine up for awhile
An externally hosted image should be here but it was not working when we last tested it.
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