Diva Audio M7 Preamp - Latest update

For all you Diva Audio M7 users out there, T.S. Lim has just revised his flagship M7 preamp.



The latest revision combines two stages of modifications.

The first stage involves new voltage supply filtering on both the B+ and the heater, combined with star ground wiring arrangement.

The second stage is a rewiring of the tube arrangement to allow easier tube rolling, and the installation of a new internal bracket that allows higher quality smoothing capacitors to be installed.



More on the tube rolling - previously the tube stages in the M7 are arranged as follows, the bottom tube uses one triode (the 12ax7 is a dual triode) as first stage, and the other triode as the second stage of the left channel. The top tube is identical and is used for the right channel. This then passes through the third stage in the middle tube (one triode for each channel). This means that you can only roll two different type of tubes at the most since the first and second stage use the same tube.



The M7 is now wired with each stage using one triode in the tube, i.e. the bottom tube is the first stage, the middle tube is the second stage and the top tube is the third stage. Now you can roll tubes to your heart's content.

Anyway, enough talk about the technicals. How does it sound ? I only managed to put in an hour of listening before sleeping but here are my initial thoughts on the latest incarnation of the M7 :-

1. Resolution - There is a noticeable increase in low level resolution and background "blackness". For example, on guitar notes, you can hear more of the body resonance of the guitar instead of just the plucked strings. Even in loud and complex passages, the increased level of detail is obvious.

2. Bass - Bass is substantial with more depth and extension.

3. Soundstaging - More 3D like with an noticeable increased in soundstage depth. Image sizes also are slightly larger.

4. Dynamics - The dynamic contrast between the loudest and softest passages is increased. Interestingly even at very high volumes, the dynamics are realistic and never fatiguing.

Note that my M7 uses a vintage military choke and NOS silver plated internal wiring. The volume pot is the standard Alps blue model though.

Oh yes, most importantly, T.S. Lim is offering this upgrade to existing M7 users for S$ 700. If you have already done the voltage filter and star ground, then the cost is S$300.

If you take this as an unreserved recommendation to upgrade your M7, you are not wrong. I highly recommend this upgrade. In current times when sought after NOS 12AX7 has reached silly money levels, this is money well spent.

An Afternoon of Tuning

Saturday afternoon was spent over at a friend's place listening to his newly built Fostex FE168 Sigma in backloaded horn cabinets and the Diva Classic 1L preamp.



My friend has the honour of owning the very first set of the 1L preamp fresh from T.S. Lim's hands.

The Diva was paired with a modded Cayin A-50T. The Cayin does not have a direct preamp in socket, so the volume knob was either turned to maximum or about 12 o'clock (at times we felt the gain was too high with the knob at maximum). The Cayin is basically a power amp with a passive preamp stage inside a single chassis.

The modifications done to the Cayin A-50T include a change of input coupling capacitors to Jensen aluminum paper-in-oil capacitors. The other coupling caps downstream were changed to Sonic Cap MkI and the feedback caps changed to Solen. The grid stopper resistor has also been changed to Japanese Kiwame resistors.

The 1L has a beautiful yellow faceplate and uses an outboard solid state rectified power supply. The preamp circuit should be very simple (we didn't open it up) since there is only a single 6SN7 tube.

The Fostex was still running in, and we felt that although the tonal balance is quite nice, there was no bass, and very limited high frequency extension. Midrange definition also lacked some texture. Probably more extended run in would be needed before final conclusions could be made.

The full range driver strengths in terms of dynamics and coherence were all there, just that the lack of frequency extremes (this Fostex driver has no whizzer cone BTW and has an intriguing flower like pattern on its cone) and slight shoutiness made the sound a bit uneven.

First up, we rolled some of the tubes. The first to go were the small tubes in the Cayin. The 12AX7 and 12AU7 used there were JJ 12AX7/ECC803 and stock Cayin 12AU7 (almost certainly Shuguang). The 12AX7 tubes were replaced with RCA 5751 double mica blackplates and the 12AU7 replaced with RCA cleartops.

Flick the switch and hummmmmmmmm. Oops. Turned out the 5751 tubes were being a bit naughty. So, the JJs went back in. Wow, what a world of difference just with the change of 12AU7 tubes. Midrange was much more defined and the overall feel was more lifelike, focused and textured.

Other adjustments done included replacing the EL34 tubes of the Cayin with EH6CA7 tubes, and the stock EH 6SN7 in the Classic 1L which was replaced with a RCA GTB blackplate. Speakers were moved out a bit and toed in almost directly to the listener.

Ah... so much better now, although we both agreed that the very creamy and rich sound of the RCA 6SN7 GTB may be a bit too slow and warm for some.

My friend suggested we try the setup with another pair of speakers, Totem Sttaffs. Turned out to be the best sound of the day, but only with judicious repositioning of the speakers in the room.

The speakers were brought out far into the room and with slight toe in aiming to a point behind the listener position.

The bass was still slightly uncontrolled and a pair of socks had to be sacrificed to block the rear-firing ports of the cabinet.

Actually we had both written off the Totem Sttaffs before this point as my friend had quite a lot of problems with the Totem that sounded very dark with soundstaging and imaging problems. But he is now keen of keeping the Totem and fine tuning it to get the best sound possible.

It was a very fun two hours and we both learnt quite a fair bit. The tube rolling was quite a shock for both of us. Although I have always been a proponent of NOS preamp tubes, this is the first time I have heard such a day and night difference with just a single tube roll.

It will be interesting to see how the Fostex turns out after another 50 to 100 hours of run in.

Quick and dirty comparison of 12AX7 and compatible tubes

Here is a quick comparison of 12AX7 and 5751 tubes in my Diva Audio M7 tube pre-amp.

For those unfamiliar with the Diva Audio M7, it is a fairly popular tube pre-amp designed and built in Singapore by Mr. T.S. Lim. It is based on the Marantz 7 circuit and uses 3 12AX7 tubes.

Here are the observations of rolling the first tube position. Electroharmonix 12AX7 gold is used in the second tube position while the RCA 5751 blackplate (double mica) is used in the third tube position,

1. GE JAN 12AX7WA (1986 date code)

The tonal balance is dense with decent midrange and treble detail. However, dynamics seem a bit flat.

2. Golden Dragon 12AX7

The tonal balance is lightweight. A bit of a paradox, but the tube avoids brightness despite quite a lot of treble extension. Midrange and treble seemed smoothed out but the highs seem to go a lot further. I say it it a paradox to me, because this kind of extended response usually results in a fairly bright presentation. The lightweight sound probably comes from the lack of bass. This tube was a lot better than what I expected.

3. Sylvania 5751 gold pin (tripe mica)

My favourite tube of the lot. A lot of detail with both extended bass and treble. Very good air with good control of sibilance. Tonal balance is close to neutral.

4. RCA 5751 blackplate (double mica)

Dark tonal balance. Very smooth and creamy but compared to the other tubes, some midrange and treble detail is lost. I suppose its quite ideal in bright systems, but match with care. That being said, I've had very good results for this tube in other equipment.