Hi Kevin!
Sorry for late reply been away for a few days!
Agreed I think the transformer is OK - it gets warm, but doesn't hum or buzz and certainly doesn't appear to have any performance anomolies. Probably worrying over nothing.
As for test equipment I only have a (pretty good) DMM. I'm on the 'novice' side of the fence really - I can read circuit diagrams and build kits from scratch which have all worked to date, but fault finding isn't an area I have had much experience with. Do you know of any scopes which are worth me keeping an eye out for on Ebay which won't cost me a limb?
Haha gotcha regarding the tubes rule thing
I will have a Dynaco ST-70 and a pair of fully rebuilt Quad IIs at my disposal sometime this year (aka when I can afford to finish them!) but just wanted to use it with a solid state power amp for the time being.
Not sure if there IS a fault or not though - as I said it never hummed when I used it with a Quad 303 couple of years ago, AS LONG AS I used it with the phono leads which had an in-built potential divider network soldered into the phono output plugs on the interconnects. Silent as a whistle then (or below audibility at any rate).
- John
Sorry for late reply been away for a few days!
Agreed I think the transformer is OK - it gets warm, but doesn't hum or buzz and certainly doesn't appear to have any performance anomolies. Probably worrying over nothing.
As for test equipment I only have a (pretty good) DMM. I'm on the 'novice' side of the fence really - I can read circuit diagrams and build kits from scratch which have all worked to date, but fault finding isn't an area I have had much experience with. Do you know of any scopes which are worth me keeping an eye out for on Ebay which won't cost me a limb?
Haha gotcha regarding the tubes rule thing
I will have a Dynaco ST-70 and a pair of fully rebuilt Quad IIs at my disposal sometime this year (aka when I can afford to finish them!) but just wanted to use it with a solid state power amp for the time being.Not sure if there IS a fault or not though - as I said it never hummed when I used it with a Quad 303 couple of years ago, AS LONG AS I used it with the phono leads which had an in-built potential divider network soldered into the phono output plugs on the interconnects. Silent as a whistle then (or below audibility at any rate).
- John
MC7R very similar to the famed Wada Shigeru mod of the Marantz 7
Anyone notice the bloated bass of the MC7R?
I bet it has something to do with the value of the feedback loop capacitor being too low in the MC7R.
The MC7R looks very much like the Wada Shigeru SRPP mod of the marantz 7.
http://www.hiend-diy.com/sch/PreAmp/hoten.pdf
or http://diagram.weeqoo.com/2007/10/2007101716483513539.html
Wada published the circuit in 1960 but it didnt gather attention till the interest in vintage Marantz 7 preamps started to heat up.
Anyone notice the bloated bass of the MC7R?
I bet it has something to do with the value of the feedback loop capacitor being too low in the MC7R.
The MC7R looks very much like the Wada Shigeru SRPP mod of the marantz 7.
http://www.hiend-diy.com/sch/PreAmp/hoten.pdf
or http://diagram.weeqoo.com/2007/10/2007101716483513539.html
Wada published the circuit in 1960 but it didnt gather attention till the interest in vintage Marantz 7 preamps started to heat up.
2w is fine.. Anyone updating this thread ???
I'm in the process of mod my preamp using the suggesting in this thread. I'm replacing all signal caps with Russian PIO caps and Russian teflon caps for 0.022uf (4). I also replaced the 4 Nichicon power caps (470uf/450v) with 470uf/450 Rifa caps and 1000uf/400v Panasonic caps in the B+ rails. I'm using 0.1uf Russian PIO caps as bi-pass for those 4 power caps.
Maybe someone @ MingDa follows this thread cause this is a recent version bought 1 year ago by a friend of mine.
Very low noise (hiss) at zero volume but still too much gain.
Would someone suggest how to reduce the gain please?
Very low noise (hiss) at zero volume but still too much gain.
Would someone suggest how to reduce the gain please?
An externally hosted image should be here but it was not working when we last tested it.
How much is "too much gain"? It should only be x8 or x10 by looking at the circuit.
1/ You can increase the NFB by reducing the value of the 15k feedback R
2/ I would prefer to bypass the first triode, taking the grid of triode 2 to the volume control, through a 1k R, and not the CRAZY 220k R there now. (These insane Chinese designers - don't they understand anything?) and of course disable the NFB loop.
If the gain is still too high, I'd then lower the 330k that feeds back from the output to the cathode of the 2nd triode.
Good hunting!
Allen
1/ You can increase the NFB by reducing the value of the 15k feedback R
2/ I would prefer to bypass the first triode, taking the grid of triode 2 to the volume control, through a 1k R, and not the CRAZY 220k R there now. (These insane Chinese designers - don't they understand anything?) and of course disable the NFB loop.
If the gain is still too high, I'd then lower the 330k that feeds back from the output to the cathode of the 2nd triode.
Good hunting!
Allen
pretty loud humming from mc7-r preamp
do not know what to say about yours but i have this problem on my amp which have had for 1 year now, and the noise seems to be getting louder ,anyway i changed tubes ,but to no effect except better sound. but when i put my hand over or on top of one of the external capacitors the noise drops alot,. i am thinking this is an earth problem ? could anyone help.thanks.
do not know what to say about yours but i have this problem on my amp which have had for 1 year now, and the noise seems to be getting louder ,anyway i changed tubes ,but to no effect except better sound. but when i put my hand over or on top of one of the external capacitors the noise drops alot,. i am thinking this is an earth problem ? could anyone help.thanks.
First, i found recently that my PSU suffer poor design.
The first cap after the 5Z3 was 150uF 400V (Philips), well datasheet tells to not exceed 25uF.
With stock 150uF cap value the rectifier has a shorter life.
Second, measured randomly various caps and most were 60/70% under the nominal value. Suspect a china fake here.
The first cap after the 5Z3 was 150uF 400V (Philips), well datasheet tells to not exceed 25uF.
With stock 150uF cap value the rectifier has a shorter life.
Second, measured randomly various caps and most were 60/70% under the nominal value. Suspect a china fake here.
I went ahead and replaced all of the stock caps with either oil or better caps. Chinese products aren't that great.
Just to make sure I have got the total list of changes correct:
1) Remove 100uF cathode bypass cap from first 12AX7.
2) Replace 270k plate load resistor to first 12AX7 with 100K.
3) Replace 100K dropping resistor to same with 270K.
4) Remove 2M2 resistor off first 0.022uF cap.
5) replace 1M resistor (adjacent to the 2M2 resistor) with 270K.
6) Remove 330K positive feedback resistor.
I assume the 22k resistor stays unchanged?
I have attached a pic of changes to make sure I have all this correct - I'd be grateful for a confirmation before I bugger up the Ming da
Many thanks!
- John
hello sorry for the complete beginner's question but does this mod have to be applied to both sides of the circuit or just one? meaning would i have to perform the above steps twice or just once? this will be my first ever mod and i haven't even started to figure out how to read the diagram yet. how nervous!
Yes, it looks like it would have a rising response below 15Hz, and significant phase shift before you get that low. Might be good for reggae or organ recitals? In addition, the forward gain path has an LF pole (0.022uF etc.) at more or less the same frequency as the feedback path, so it only takes a bit more phase shift somewhere to cause subsonic oscillation. Even if it doesn't oscillate, it will jump about due to mains variations or arm resonance if someone is brave enough to use it after a phono preamp.
The 2k2 resistor in the first stage cathode may generate more noise than the valve. This could be noticeable at the low volume settings caused by the excessive gain. The first stage after the volume control should be designed for low noise.
I'm astonished that such a poor design is actually being commercially sold. Why generate all that gain only to throw it away? There are good reasons for doing this in a power amp, although some would not even do it there. Much less reason to do this in a pre-amp, when in most cases no gain at all is needed. Because of all the feedback the second stage has to have a sharp HF cutoff due to an overly large grid stopper - looks like a guitar amp design! This means that the overall HF response may depend too much on the exact Cag of the valve used - scope for tube rolling.
Why do people use 12AU7 as a cathode follower? It has low gm and low mu, so quite unsuitable. I suspect it may be because 12AU7 has low output impedance in common cathode, because of its low mu, and people just assume it is a 'low impedance' valve. In CF duty you need high gm to get low impedance and low distortion. I would not tinker with this design, but 'improve' it by using the chassis and PSU for something else!
hello sorry for the complete beginner's question but does this mod have to be applied to both sides of the circuit or just one? meaning would i have to perform the above steps twice or just once? this will be my first ever mod and i haven't even started to figure out how to read the diagram yet. how nervous!
Both channels need to be changed as noted. Go back and read my old posts very carefully.
Hi,
i have this preamp with all of these mods but i used mundorf silver/oil caps, sonicap gen II, obbligato caps, solen caps and kiwame resistors. I bypassed all PS caps. Tyffany rca plugs from a Classé DR6 preamp.
Then i bought one RCA vt244, 2 cv4004 Brimar and 2 cv491 Brimar......now it sounds very good.
My current "problem" is that i bought a Gryphon S100 amplifier at a very low price and it has balanced input only, so i'm using rca to xlr neutrik adaptors but this is only a momentary and not the best solution.
I love the sound of this preamp.............incredible sound very detailed,warm and dark etc etc, one of the best sound preamp that i ever heard!!!! so i want to modified its outputs from RCA to balanced and i want to use my actual output tubes (Brimar cv491 one of the best and expensive 12au7).I read about "Phase Splitter" to change in balanced output but i don't know how to start because i'm a tube "beginner". Anyone can help me,please? I need only some directions to start and to know what are the problems that i must to solve....then i wish to have a really little Zout.
If it's too difficult to change current output, I thought to use Jensen audio isolator, (a box with rca to xlr with trasformers with very good frequency response, 350KHz). But i wish to use my tubes without any km copper wire of trasformer.
In the first post there is a schematic, even if not the last one with mods.
thanks
Gaetano
This thread on Hifisentralen describes how to easily convert the MC-3 to 12B4. I suppose that the same technique can be used with the MC-7R.
DIY linjetrinn; Fidelity K.I.S.S.
The power amps from MingDa/Mexing often have very low input gain, hence the need for their preamps with very high gain.
DIY linjetrinn; Fidelity K.I.S.S.
The power amps from MingDa/Mexing often have very low input gain, hence the need for their preamps with very high gain.
Hi and thanks a lot!
but what i'm looking for is how to convert rca output in XLR output (balanced output).
I began to read the tread that you telle me but it seems to speak about only rca output
Hello, would some one be able to show me a picture or diagram of the capacitors in the signal path of a ming da mc-7 pre amp please.
I am new to electronics modification but I am fairly skilled with a soldering iron after many years of racing electric rc cars.
The reason for asking is I wish to change the specific capacitors in the signal path to v cap teflons.
Thanks .
I am new to electronics modification but I am fairly skilled with a soldering iron after many years of racing electric rc cars.
The reason for asking is I wish to change the specific capacitors in the signal path to v cap teflons.
Thanks .
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