I'll try that later tonight when I get home.
The last schematic posted shows the heater winding at ground potential as both 100 ohm resistors go to ground.
Aside from the slight noise coming from the tube buffer, the amp is basically dead silent.
I find it interesting how some who restore Magnavox amps use different coupling cap values so that the low frequency phase shift doesn't cause bass instability.
I took a much different approach. I simply eliminated the phase shift as if the phase shift isn't there it cannot contribute to bass instability.
There any benefit to putting a metal bottom on the amp?
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You certainly won't hurt anything to shield the bottom. I put my Maggies on an oak plinth, with a sheet of flimsy sheet metal between as an electrical shield.
I forgot to "close the loop" on the comment I was making earlier about increased amplitude as you approach OPT resonant frequency- The Maggie stock transformers had a pretty low resonant frequency, preventing correction with a capacitor across the feedback resistor, as it would cause negative effects within the audible range, requiring the use of the Magnavox hobbled phase inverter for a HF roll-off to compensate. The way Dave fixed this on the 9300 (after a much improved floating paraphase inverter) was to take a HF feedback signal from the upper driver plate, cutting the OPT out of the high frequency feedback loop.
Snipping from post #73:
"3. NFB/STABILITY: The approach to stabilizing the NFB employed now includes a new active HF compensation loop in the form of a 47 pF cap connected between the plate of the top output tube, and the NFB insertion point at the cathode of the AF amplifier stage. With this additional loop in place, the overall closed loop gain of the entire amplifier is actively controlled at HFs, and since the OPT is not included in this loop, it effectively reduces the HF response of the amplifier so as to be under that of the resonant frequency of the OPT, and thereby eliminating the ring associated with the original design. Used in conjunction with a new step network added to the plate of the AF Amplifier stage, HF stability is greatly improved, with the amplifier now being able to handle capacitive only loads of at least .03 uF. Also, with the dual feedback loops, frequency response is no longer elevated above 10 kHz as in the original design, but now down just .25 db at 20 kHz.
On the low end, the coupling caps have been chosen to intentionally limit response below 40 Hz, so as to minimize saturation of the OPT. But even at this, response at 20 Hz is down only 2 db, being 1 db down at 40 Hz. This represents better than an order of magnitude improvement over the original design, and response that is still better yet than those modifications that use .01/.1 combinations with the stock design. This is possible because of the improved LF stability that the floating paraphase inverter design offers, with the amplifier still showing rapid settling under pulsed conditions."
I forgot to "close the loop" on the comment I was making earlier about increased amplitude as you approach OPT resonant frequency- The Maggie stock transformers had a pretty low resonant frequency, preventing correction with a capacitor across the feedback resistor, as it would cause negative effects within the audible range, requiring the use of the Magnavox hobbled phase inverter for a HF roll-off to compensate. The way Dave fixed this on the 9300 (after a much improved floating paraphase inverter) was to take a HF feedback signal from the upper driver plate, cutting the OPT out of the high frequency feedback loop.
Snipping from post #73:
"3. NFB/STABILITY: The approach to stabilizing the NFB employed now includes a new active HF compensation loop in the form of a 47 pF cap connected between the plate of the top output tube, and the NFB insertion point at the cathode of the AF amplifier stage. With this additional loop in place, the overall closed loop gain of the entire amplifier is actively controlled at HFs, and since the OPT is not included in this loop, it effectively reduces the HF response of the amplifier so as to be under that of the resonant frequency of the OPT, and thereby eliminating the ring associated with the original design. Used in conjunction with a new step network added to the plate of the AF Amplifier stage, HF stability is greatly improved, with the amplifier now being able to handle capacitive only loads of at least .03 uF. Also, with the dual feedback loops, frequency response is no longer elevated above 10 kHz as in the original design, but now down just .25 db at 20 kHz.
On the low end, the coupling caps have been chosen to intentionally limit response below 40 Hz, so as to minimize saturation of the OPT. But even at this, response at 20 Hz is down only 2 db, being 1 db down at 40 Hz. This represents better than an order of magnitude improvement over the original design, and response that is still better yet than those modifications that use .01/.1 combinations with the stock design. This is possible because of the improved LF stability that the floating paraphase inverter design offers, with the amplifier still showing rapid settling under pulsed conditions."
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As 20to20 said, sweep it for different properties. 10Khz is the common choice for evaluating overshoot ring as it is a good standard to compare to pictures from other amplifiers, and it should resolve well before the downward transition. Very low frequency to see how coupling caps are discharging and if the OPT is saturating. Don't worry if/when you see the corners start to round off above 10Khz. If you read up on Fourier representation of a square wave, it takes up to the 5th to 7th harmonic to get it to look like a square wave with squarish corners, so if your amp rolls off at 40Khz, it has stellar audio performance but the 10KHz square wave will look pretty rounded.
Wouldn't it make more sense to eliminate the problem which is the output transformer?
I can see why most do not do that because the transformer costs a good bit more than a few components and it also takes away from the originality.
I can see why most do not do that because the transformer costs a good bit more than a few components and it also takes away from the originality.
When the problem is how to create a mono program into a small bedroom that does not have enough the space to set up a stereo of speakers.
Op Amps can be wonderful tools. In this case, seems like unity gain stages will suffice.
Unity gain unfortunately won't be good enough. Must have at least a gain of 2 given the preamp is passive.
On the 8600 single ended build the first thing he did is dump the transformers in favor of 8 Ohm to maximize the power transfer from the tubes to the speaker because it was a flea power amp and could not tolerate the 1/2 power loss. For the 9300, the intent of the first stage of the build was to keep within the confines of the original chassis and see what he could do. I'm sure cost was a factor there, and I appreciate that because I like to keep vintage equipment as stock as possible. Even though the transformers were 4 ohm, the Enhance Fixed Bias allowed such a lean operating point with so much clean headroom that you don't see the full 1/2 power loss in the impedance mismatch- He gets a solid 9W into 8 ohms, 15W into 4 ohms, with very low distortion figures (whereas the stock design had gross crossover notch and huge harmonic distortion well before 10W into 4 ohms. Most of my speakers are down in the 6 ohm range, and the performance is stellar, so I never entertained the thought about changing the transformers. (Plus I like those 1961 Stancor transformer numbers looking back at me.) Others rushed to new transformers right off the bat, thinking that was some kind of bromide while neglecting huge deficiencies elsewhere in the amp, and without the knowledge of how to properly tune the feedback. Dave does swap the transformers later in the thread, which is about where I lost a lot of interest.
I would certainly add gain, a small amount, maybe trimmer adjusted so you can both match the two channels together (to better tolerance than a dual pot) and to match the volume knob position on the pre-amp to the loudness you want from the speaker. Nothing like a volume control that has power, yet retains subtle control over lower levels. The op-amps have such a good low output impedance that even with a series resistor to enable it to better handle driving a capacitive load, it will be well low enough for the run and the 47K input impedance. The op-amps draw so little power into that load, and have such a good power supply rejection ratio, that the dual single-wave rectified schematic you posted earlier should be nice and quiet. Use a transformer based AC wall wart if you can find one rather than a switcher- Geez have I come to loath those.
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Unity gain, 2x gain or a reasonable level of gain, pretty much irrelevant.
Reading through this thread, if it were my project, I would start off simple basic summing amp to create the mono, add a stage of gain if I feel I need it. OP Amps are pretty easy to work with, use OPAmps for this job.
I would make a box that sums L & R, with the stage of gain. If you are looking to introduce some "tubi-ness" you can make a two boxes. One box as a summing circuit, at the source end. Add a basic tube gain stage at the speaker end.
I usually do as well unless I need to fix a fault of the design or make a reasonable improvement to a good design.
I've known about the DRV134 chip for quite some time and always wanted to use it for something but never had the chance. Think I'll use it here.
https://www.ti.com/lit/gpn/DRV134
So if the built in 50 ohm resistors are enough to allow me to connect the outputs of two chips together I can use a balanced 1/4" jack and when I insert an unbalanced 1/4" plug I'll still get the gain of 2, however if I need to drive a balanced load I can insert a balanced 1/4" cable and the output will be balanced.
What I may do is use a DPDT switch and add a second balanced 1/4" jack so that I then have the option of stereo or mono output. I can use the toroidal power transformer I used for the tube buffer and can likely use the same case given it's a computer power supply case. The metal will provide good shielding.
I've got a Schitt SYS at work that I originally got because the amp I use with my speakers only has a few volume steps a lot like a bluetooth receiver module does and needed it to control the volume better.
https://www.schiit.com/products/sys?srsltid=AfmBOooKu80rWM04XvL4L7dZLzq4JEbBvYJlGDVNzHbMzJ7y82-viPIv
Anyways it has a small volume knob which is not suited for use as a volume control in my opinion. So I bought another passive preamp off Amazon that has a properly sized volume knob.
Whatever I go with whether it be the DRV134 board or my own design, I'll use the SYS before the buffer to set the audio level so that the Saga S volume control is not too sensitive.
I do like the DRV134 board though so I may well go with that one given I can have a balanced and unbalanced output depending on whether I use a balanced or unbalanced cable.
I bought this one.
https://www.amazon.com/2-Channel-Un...ctronic-Performance/dp/B0F1LSYXPL/ref=sr_1_11
https://www.schiit.com/products/sys?srsltid=AfmBOooKu80rWM04XvL4L7dZLzq4JEbBvYJlGDVNzHbMzJ7y82-viPIv
Anyways it has a small volume knob which is not suited for use as a volume control in my opinion. So I bought another passive preamp off Amazon that has a properly sized volume knob.
Whatever I go with whether it be the DRV134 board or my own design, I'll use the SYS before the buffer to set the audio level so that the Saga S volume control is not too sensitive.
I do like the DRV134 board though so I may well go with that one given I can have a balanced and unbalanced output depending on whether I use a balanced or unbalanced cable.
I bought this one.
https://www.amazon.com/2-Channel-Un...ctronic-Performance/dp/B0F1LSYXPL/ref=sr_1_11
I have to say, your thought process and logic is different than mine.
I would install a tested and proved typology. Build and test it with basic op amps. If it works using TL-072 devices, you can substitute the high performance devices of your choice.
I often tend to build things myself, however looking up the specs on the DRV134 chip, it looks like just what I need, however the only thing I need to be concerned about is if the chips on the board are legitimate or fake.
No problem to follow the logic (I think): so when gain is not enough you can use a tube summing preamp with gain but high Zout after the Schiit so before the DRV134 board (maximum 17 Vrms) and if then gain turns out to be a tad too much you can attenuate at the input of the power amplifier. If you add volume control to the power amplifier to attenuate you can fine tune volume at 2 spots too.
If the DRV134 turn out to be fake you could order 2 original ones and then replace them.
If the DRV134 turn out to be fake you could order 2 original ones and then replace them.
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The gain of 2 will be slightly higher than the 1.8 something gain I measured so if anything I may need to use the Schiit SYS to lower the level some. going into the DRV134 board. The benefit there is should I find another set of drivers and another Magnavox AMP-142 I could go stereo and only have to flip a switch on the box the DRV134 board will be mounted in. Plus I can switch to a balanced output by just plugging in a balanced cable.
Step us back to why the Saga isn't doing what you need? Once you got the Amp-142 input Z down to 47K (it could even go lower now that we know the Saga has a low output Z)and the noise gone, if you don't want to use the SS gain section of the Saga but you want to build a SS gain buffer...? You can create a channel sum on the 142 input like you did for the buffer.
The Saga only has a gain of 1 in active mode and the gain I measured of the tube buffer is 1.8. So the gain of 2 will be just right. Of course I could alter the feedback in the AMP-142 to make up that gain, however that will require a lot of testing and experimenting with compensation cap values to get the response as flat as reasonably possible.
Maybe you can confirm true or not the spec of 5vRMS max output from the Saga. Does that mean 5v in to get 5v out? Gain of 1.
When I read the specs of a preamp that says max output, that means signal voltage at the outputs with the volume control at max and a strong input level anywhere up to 2v. So, considering that the common source devices out there like DVD players that put out 2v, to get 5v the gain would be 2.5.