HT MC Phono Stage AD797

[Hiten]

Wow ! That's pretty good for a camera Phone. It has even captured some reflection on Paper clip !!

 

[jls001]

Brief update:
The sound of the AD 797 phono pre is improving. Initially bass was less and whatever was there was bloated. It has now become nice and tight after playing many record sides. One thing that was apparent from the first drop of the needle was the quietness of the phono stage. This allows one to play (quite) a bit louder (even 6 dB louder) without listening strain. For a pedestrian setup like mine, this is a blessing as it brings out more micro details.

Gain setting: I had reported earlier that the gain setting I had set was quite insufficient for the Denon DL 103. I remember setting both GainL and GainR pots to about 1000 Ohms but last night when I rechecked, both were only 741 Ohms. I increased the pot value to the maximum possible (till the click of the pots) and the max resistance was only about 830 to 834 Ohms. This is surprising as the pots are supposed to be 2000 Ohms Bourns pots. Could it be that the Bourns are by mistake 1000 Ohms, and not 2000? I need about 1200 Ohms resistance to achieve my desired loudness.

To add: I added a 2 mm thick mild steel spacer in the headshell to give the tonearm more mass (the Denon 103 needs higher mass). The introduction of the spacer also gives better VTA. Which reminds me I need to fashion a new armboard, 9 or 10 mm thick, unlike the current 12 mm thick one.

 

[sachu888]

Brief update:
The sound of the AD 797 phono pre is improving. Initially bass was less and whatever was there was bloated. It has now become nice and tight after playing many record sides. One thing that was apparent from the first drop of the needle was the quietness of the phono stage. This allows one to play (quite) a bit louder (even 6 dB louder) without listening strain. For a pedestrian setup like mine, this is a blessing as it brings out more micro details.

Gain setting: I had reported earlier that the gain setting I had set was quite insufficient for the Denon DL 103. I remember setting both GainL and GainR pots to about 1000 Ohms but last night when I rechecked, both were only 741 Ohms. I increased the pot value to the maximum possible (till the click of the pots) and the max resistance was only about 830 to 834 Ohms. This is surprising as the pots are supposed to be 2000 Ohms Bourns pots. Could it be that the Bourns are by mistake 1000 Ohms, and not 2000? I need about 1200 Ohms resistance to achieve my desired loudness.

To add: I added a 2 mm thick mild steel spacer in the headshell to give the tonearm more mass (the Denon 103 needs higher mass). The introduction of the spacer also gives better VTA. Which reminds me I need to fashion a new armboard, 9 or 10 mm thick, unlike the current 12 mm thick one.

Hi Joshua,
There are 2X1k and 2X2k Bourns trim POTs.I think 1k for resistance and 2k for gain.
Hypnotoad says 500Ohms are enough for most carts,please see his post #7 here http://www.audiokarma.org/forums/showthread.php?t=501186
EDIT: There should be no 202 printed on 2k trimmers.1K trimmers should have 101 print on them.

Regards,
Sachin

 

[sann]

I added a 2 mm thick mild steel spacer in the headshell to give the tonearm more mass (the Denon 103 needs higher mass). The introduction of the spacer also gives better VTA. Which reminds me I need to fashion a new armboard, 9 or 10 mm thick, unlike the current 12 mm thick one.

Hi, Could you please post a pic of this 2mm ms spacer? is it sourced or a DIY? does this require additional counter wight to balance the arm? Regards, sann'

 

[jls001]

There are 2X1k and 2X2k Bourns trim POTs.I think 1k for resistance and 2k for gain.

Hmmmm.... does this mean I have swapped the input impedance adjustment pots with the gain adjustment pots? Need to check Thanks for pointing out.

 

[jls001]

Hi, Could you please post a pic of this 2mm ms spacer? is it sourced or a DIY? does this require additional counter wight to balance the arm? Regards, sann'

I'll take photo. This is not DIY, but if you have a means to hold down the sheet, it should be possible to drill the holes exactly half inch apart. Now, I don't even remember where I got it from:annoyed:

 

[jls001]

Hmmmm.... does this mean I have swapped the input impedance adjustment pots with the gain adjustment pots? Need to check Thanks for pointing out.

The pots are in their correct places. What was wrong was the method of measurement. One must remove the opamps from the circuit while adjisting and measuring the resistance of GainR and GainL pots.

I set the resistance at 1700 Ohms. With this, the gain is much better than before but still about 6 dB short of comparable CD level. I will stick to this now as line level preamp can provide the needed volume.

 

[HypnoToad]

Around 1200 - 1300 ohms is needed for the Denon DL-103. There is no need to remove the op amps.

You measure between the closest legs of the resistors to the trimmers, R15 & R9 for the right channel and R16 & R10 for the left channel.

It is possible to swap the trimmers over and have the 1k in place of the 2k, which won't matter much for impedance adjustment but will not give you enough gain.

As Sachin say the right ones should have 202 on them, the P/N being 3296Y-1-202LF.

 

[jls001]

Pics of the headshell spacer:

Heavier counterweight is required to balance the additional weight of the spacer. Some people glue coins, etc to increase counterweight's weight.

 

[jls001]

There is no need to remove the op amps.

When I measure across R15-R9 (or R16-R10) with the AD797 in the socket, the max resistance I observe is 834-836 Ohms. On removing the AD797, that value jumps to 199x Ohms. With AD797 removed, I adjusted the resistance to 1700 Ohms.

When I had set it the first time (I don't recall if this was with or without the opamp), I had set it to an arbitrary value of 1003 Ohms each, which I later found was too less for my setup for the DL103. When I was about to adjust the resistance, I observed 743 Ohms (was set for 1003 Ohms), and the max that I could go up to was about 836 Ohms.

It is possible to swap the trimmers over and have the 1k in place of the 2k, which won't matter much for impedance adjustment but will not give you enough gain.

I tried desoldering the 202s but the wick let me down

As Sachin say the right ones should have 202 on them, the P/N being 3296Y-1-202LF.

It is indeed marked "202". The input trimmer is marked "102".

 

[HypnoToad]

I tried desoldering the 202s but the wick let me down

Don't desolder them they are in the right spots.

I just went and checked one of mine I just populated and the resistance is the same whether the AD797 op amp is installed or not.

Here is proof:

With Chip -

Without Chip -

Closeup of connections -

 

[jls001]

OT-ing this a bit:

The fabulous/infamous/notorious (please take your pick) Denon DL 103 is reputed to have a very low compliance and therefore needs a tonearm with high effective mass to play at its best. The published compliance being 5 x 10^-6 cm/dyne (100 Hz). There is a section of audio enthusiasts that insists the actual measured compliance is something like 9 x 10^-6 cm/dyne (100 Hz), which is a lot higher than the manufacturer's claimed compliance. Tonearm effective mass of something in the range of 16-18 grams or higher is considered essential to produces a resonant frequency of 9 to 11 Hz. That's the abridged theory. Or the version that I understand:lol: There could be other truths out there:lol:

The effective mass of my tonearm (Origin Live Silver Mk IIIa) is about 14.5 grams. In order to try and increase the effective mass of the arm, I had added a headshell spacer (see post # 107 on this thread). The mass of this spacer is 3.29 grams. After fitting this spacer, I found one more spacer among my audio knick-knacks which weighs 1.43 grams, so I added that too, increasing the mass on the headshell end by 4.72 grams.

I thought one could go one up and use an even heavier spacer. Since I didn't have anymore spacer, I decided to DIY, and last night I wrought something that weighed 5.97 gms. The final mass could have been a wee bit higher but I was forced to make it thinner as the longest headshell screws I have at hand would not hold it. The final thickness is nearly 3 mm. As can be seen from the pictures, I didn't bother too much about the finishing as this is just for proof of concept. It was only later on fitting the new spacer that I realised that I have reached the limits of what the counterweight could balance.

Some pictures:

1) the raw material - a C section, which I thought was chrome plated mild steel. On cutting it, it turned out to be brass. And a very interesting piece of brass it turned out to be - one section was 2 mm thick, a second section 3 mm thick and the third section was 4 mm thick! I chose the 3 mm thick section as I already had a 2 mm spacer.

2) the 3 mm thick portion being cut to size

3) the screw holes marked

4) Side by side with the 2 mm thick spacer (3.29 grams) and 0.5 mm thick spacer (weighing 1.43 grams).

5) Stacked for a comparo of thicknesses. Sorry about the noise in the picture. That's what one gets from a cell phone's tiny image sensor in less than optimal ambient light.

6) Mounted!

Sonic improvements: the bass is (much) more and much tighter than before. I can also hear a better focus to the sound. I also found out that the listening chair is not the most optimal position to listen to my setup

Next steps: (a) to try and make some mathematical sense out of this - as in, find the optimal weight for the spacer (for the Denon DL 103 cartridge and this arm) (b) find longer headshell screws as the current one is barely able to hold the thicker spacer. (c) try other LOMCs too.

Suggestions/opinions/critiques welcome

 

[sachu888]

Good experiment Joshua.I recently bought vintage Audio Technics AT-15ea cart.It is sounding very nice in my system,better than OM10.I don't know its compliance but it looks to be on the lower side.I think adding 3 gram spacer could do the trick.
BTW AK member Qwin just completed AD797 Phnostage.His build is one of the best in AK.You can see his build here AD 797 Phonostage don't forget to check his another projects.

Regards,
Sachin

 

[jls001]

BTW AK member Qwin just completed AD797 Phnostage.His build is one of the best in AK.You can see his build here AD 797 Phonostage don't forget to check his another projects.

What an awesome build!!!

 

[sachu888]

I also liked his Technics 1210 project.

Regards,
Sachin

 

[sann]

Thanks jls001, your DIY and experiments are encouraging. This is what I had been looking for, I have been continuously digging the internet for increasing the tonearm mass, adjusting the VTA for the low hight (will pick notorious - hard to tame) DL-103 on the TT's which dont have tonearm height adjustment, and adding aux weight to the counterweight to balance the added weight.
How the spacers available on eBay are?
Technics 3 Gram Headshell Shell Weight Spacer Genuine New Part UK Stock | eBay

Keep writing.
Regards, sann'

 

[jls001]

Thanks jls001, your DIY and experiments are encouraging. This is what I had been looking for, I have been continuously digging the internet for increasing the tonearm mass, adjusting the VTA for the low hight (will pick notorious - hard to tame) DL-103 on the TT's which dont have tonearm height adjustment, and adding aux weight to the counterweight to balance the added weight.

How the spacers available on eBay are?
Technics 3 Gram Headshell Shell Weight Spacer Genuine New Part UK Stock | eBay

Keep writing.
Regards, sann'

The spacer looks good. Judging from the size and weight, I am guesstimating that it will be about 2 mm thick. What arm do you use? Any idea what is its effective mass?

There are some constraints in trying to increase effective mass using spacers:

1) the introduction of a spacer calls for corresponding adjustment in VTA. VTA is very important to get right. It decides the tonality of the turntable. Personally, I would not recommend increasing effective weight at the cost of VTA.

2) the introduction of extra weight at the headshell end requires that one has enough length of shaft at counterbalance end to balance the increased weight. Alternately, one must have heavier counterweight, or perhaps an extra counterweight. But the better solution is to move the counterweight as far back as possible as that increases the effective weight better than just fitting a heavier counter weight.

3) the extra weight at either end must be rigidly affixed. So a spacer which is tightly screwed between the headshell and the cartridge is ideal. People also use wraps on the arm tube, but this is more likely to distribute the weight over the length of the arm than concentrating it at the end of the arm tube. This idea (that it is better to concentrate the weight at the end of the tube instead of distributing it across the tube) sounds counter-intuitive to me but that seems to be the accepted wisdom. Arm wraps also additionally serve to dampen arm tubes. If you have to tape an extra weight, do make sure that it doesn't become a source of spurious vibration. Tape it down well.

 

[sann]

I useTechnics SL-D3, tonearm effective mass is 12g, if I add around 3g at the headshell the counterweight moves back to the extreme (uncomfortable) position at the end of the arm.

 

[jls001]

The discussion here has a practical, if not completely accurate, way of measuring the effective mass of a tonearm. This formula is for the very simplistic case of a straight tone arm with a uniform arm tube (because moment of inertia depends on the distribution of mass and not just the mass), and the headshell removed to simplify the calculation (or headshell's contribution being ignored as insignificant).

Any idea what would be the formula and process to be followed for the real world situation taking into consideration the presence of headshell, screws/nuts/washers, cartridge, AND a headshell spacer?

I understand one can reverse engineer the effective mass of an arm provided one has the HiFi News and Records Review magazine's test record, and also one knows the compliance of the cartridge.

The resonant freq of my arm-cart combo (stock) is 14+ Hz as per Freek's Resonant Frequency Calculator found here, and that is hardly comforting. I need to get that down to 9 or 10 Hz.

 

[jls001]

Any idea if there is an ideal voltage to power the AD 797 phono preamp?

I had so far been using two rechargeable 9V batteries. Today I tried a different power supply of +12V 0 -12V, and I noticed an increased volume at the same volume setting as before.

@hypnotoad: is it possible that this could be the reason why I wasn't getting enough gain, whereas you had plenty?

But while fiddling around I managed to introduce hum which wasn't there before. The hum is quite audible. I managed to bring it down by rerouting the tonearm cable, but there is still some hum whereas there was almost none before. You gain some, you lose some:sad