Dedicated DIY Audio Thread

[jderimig]

Polypropylene has very low dielectric absorption. Dielectric absorption shows up as something that you might model as part of the capacitance with a resistor in series. So part of the capacitor has a long time-constant.

Were the caps PP or polyester (greencaps)? Polyester is much worse for DA.


There might have been a subtle shift in the RIAA curve causing that. What tolerance were the caps and resistors in the network?

Pretty sure they were pp. Don't know what the tolerances were but she mentioned they need to be good. I don't think she would have used poor choices there. I would have to watch the vids again.

 

[OverTheTop]

Most caps are 5% or 10% which is waaay too wide for RIAA. Need to either select values or spend extra on tighter tolerance units.

 

[tsmith1315]

Most caps are 5% or 10% which is waaay too wide for RIAA. Need to either select values or spend extra on tighter tolerance units.

Quote from Nelson Pass about resistor tolerance:

"I use 1% unless I'm trying to land a probe on Mars or make RIAA EQ"

 

[jderimig]

Got the board today. Bigger than I thought. Looks very nice. The board is supplied with resistors and power supply components. One nice touch is that the resistors are placed on the board but not soldered. This allows checking resistance values before final soldering. One lesson learned is to order the kits with all the components. The kit I got did not include the caps to allow the user to substitute "boutique" caps. You can get the full kit with all components for about $70 or fully assembled for the same cost. Then tweak with better components where they might be useful, like the coupling caps. I priced the caps (not fancy) from Mouser to finish the board, the cost would be around $135!. So I ordered another full kit for $70. Then I will use orange drops and other caps I already have and sub in and experiment. The RIAA caps are 1% silver mica's.

 

[jderimig]

Finished populating the board. Stuck a tube in just for show. The straight razor in the pic is left for your own artistic interpretation. The power supply part of the board (right) is detachable from the rest of the board in case some distance between PS and amp is desired. Some have built this using 2 chassis. Soon I will put some power on it and see what happens. Applied some recommended mods from the Lenco Heaven site. Main one is to remove the rumble filter components and tweaks on the RIAA equalization components.

 

[jderimig]

Progress. Finished the preamp and had some hum with the inputs shorted. Went to a star ground arrangement like the build threads suggested and the output is dead quiet with inputs shorted. However I had lound hum with the turntable connected I have to troubleshoot. My RM5 preamp also has large hum so it looks like my tonearm/table wiring may have developed a fault after a decade of no use and a house move. This weekend I hope to get the scope out and trace out where the noise is coming from.

 

[jderimig]

Solved the hum issue. Tied the turntable ground wire to the preamp star ground point directly (not grounding through the chassis). This is dead quiet. I thought I blew something up when I turned in on as I could not hear anything. This thing is awsome sounding. I haven't done any serious vinyl listening in over 10 years and put on the Mirage album from Fleetwood Mac and it blew me away. And that is with my 30 year old Grado Black cartridge.

 

[FredA]

Good to hear the sound is awesome! Vinyl rules.

 

[tsmith1315]

Thanks for posting, sounds like a cool project. And a great price!

 

[jderimig]

Fred, yes it does. Now I got to make my concrete slab turntable platform. I have 4 bags of Quikcrete in the garage left over from yard project to use. This will use 1 bag.

 

[jderimig]

There is something magical about a MM cartridge feeding a tube phono amp with a 12AX7 V1.

 

[Greg Furtman]

Fred, yes it does. Now I got to make my concrete slab turntable platform. I have 4 bags of Quikcrete in the garage left over from yard project to use. This will use 1 bag.

Have a sketch or a pic of what your turntable platform will be like?

 

[jderimig]

Have a sketch or a pic of what your turntable platform will be like?

Not yet but I can describe the concept. I will make a mold out of melamine board. The concrete mix will be countertop mix with an additive that lowers the viscosity so the surfaces against the mold will be glass smooth. While the concrete is still uncured I will insert 4 1/2-20 threaded inserts that will be used for leveling feet that will be screwed in. The size will match my Thorens TD160 dimensions which puts the slab at 15"x17"x3.5". It will weigh about 90#. Cost ~$5. It will be colored walnut brown as I have alot pigment left over.

 

[OverTheTop]

Don't forget to make something that can vibrate the concrete to get the bubbles out. Otherwise you will have glass-smooth sides with bubbles.

 

[John Kemker]

You can find concrete vibrators online for ~$38. They also work great for vibrating propellant. [grin]

 

[Hobie1dog]

Don't forget to make something that can vibrate the concrete

ask your wife if she would have something that would work

 

[FredA]

I've found my granite slab insufficient by itself.
Although the imaging is super-solid, the bass went a bit muddy compared to the butcher block.
Despite weighing in at 150#, the slab still is affected by the subwoofer.
I've ordered up a different butcher block that's the same size as the granite slab and will be putting that on top.
I think one needs mass separated by something that flexes.
So I ordered this: Double-B-Maple Isolation Platform
Was supposed to be here Friday - Late as is usual for UPS - will be here tomorrow.

 

[Hobie1dog]

I've found my granite slab insufficient by itself.
Although the imaging is super-solid, the bass went a bit muddy compared to the butcher block.
Despite weighing in at 150#, the slab still is affected by the subwoofer.
I've ordered up a different butcher block that's the same size as the granite slab and will be putting that on top.
I think one needs mass separated by something that flexes.
So I ordered this: Double-B-Maple Isolation Platform
Was supposed to be here Friday - Late as is usual for UPS - will be here tomorrow.

Either one of them is still coupled to the floor by a rack or platform, correct? You would still need rubber or rubber cork sandwich style isolation blocks to keep the vibration from the floor from going up to the concrete or the maple block. That's just structure born vibration getting up to your turn table base.

 

[OverTheTop]

Conical rubber bumpers?

 

[jderimig]

Don't forget to make something that can vibrate the concrete to get the bubbles out. Otherwise you will have glass-smooth sides with bubbles.

I use a hammer on the sides of the molds. The viscosity additive I use is amazing. A tablespoon or two per bag makes the mix really soupy.

 

[jderimig]

I've found my granite slab insufficient by itself.
Although the imaging is super-solid, the bass went a bit muddy compared to the butcher block.
Despite weighing in at 150#, the slab still is affected by the subwoofer.

How do you know the slap is moving and not the table relative to the slab? I am not familiar with the VPI but what is the machanical path from the spindle bearing to the slab. If the table is moving the stone slab with not dampen movement of the table without some energy absorbing function in between the table and slab. Maybe that is why a butcher block works, it will.

 

[jderimig]

I've found my granite slab insufficient by itself.

You probably have all the instrumentation to know and tune your tt mounting.

1. Get oscilloscope and signal generator (or use PC sound card).
2. Turn platter off and place tonearm cartridge on a record or the platter.
3. Hook the oscilloscope to your preamp out.
4. Run the signal generator to your amp. Sweep frequency from DC to light.
5. Ideality is seeing no output from the cartridge.
6. Tweak, lather, rinse, repeat.

 

[FredA]

With a bit of wire swapping, I can run a REW sweep while looking at the preamp out.
Need to do that anyway to find and quell the resonance my sim-seat has.
Kill two birds at once.

 

[jderimig]

With a bit of wire swapping, I can run a REW sweep while looking at the preamp out.
Need to do that anyway to find and quell the resonance my sim-seat has.
Kill two birds at once.

If you are using REW then you can run the line-out of your preamp into line-in of REW and get the transfer function....or see where you need to damp.

 

[cvanc]

I get what you're trying to measure but it's more complex than that.

The mechanical system is a bit like a saxophone, where more-or-less broadband energy is applied (you blow into it) and the saxophone decides what note to make of it (what holes are open or closed). It is a resonator and it doesn't much care what frequencies the input energy is at, it only cares how much total energy there is, and it will make the note it wants to make from it, thank you very much.

When you examine low frequency signals from the cartridge, this is entirely dominated by the arm/cartridge resonance. I think your method of measuring TT output by exciting the room while sitting stationary in the groove will just show you the arm/cartridge resonance, because it will take whatever energy spectrum that impinges on it and convert it to the frequency it wants to ring at. It's another saxophone.

If you pump in enough broadband energy (music), acoustic feedback can break out. This always happens where overall mechanical gain is greatest, and that is the arm/cart resonance, every time.

TL;DR... you're trying to use the cartridge as an accelerometer but the problem is at very low frequencies it's a lousy one with a huge resonance of its' own. Nobody buys accelerometers with huge resonances right in the desired passband of use.

Examples attached.

 

[jderimig]

^ All true. But resonance is a measure of energy getting back into the system. Ideally the output should be zero. Cartridges are lousy accelerometers, but still 1000x better than your ears.

The other method is instead of doing a REW sweep, drop a bowling ball on the floor. Ideal output is still zero.

 

[cvanc]

...So where does this lead us?

You absolutely want maximum isolation from the outside world, as much as you can provide. Adding mass to the feedback path (with slabs & such) does help but it's not the sharpest knife to use.

A more head-on approach is to have another resonant system in the feedback path that is tuned BELOW the arm/cart resonance. This means very low indeed, as arm/cart systems ring at 10Hz plus or minus maybe 5Hz. This suggests external isolation systems should have an obvious low-frequency 'bounce' to them when excited.

Answers like this exist and work best of all, but hang on to your wallets. (I never said this wouldn't be painful)



Expensive!

 

[FredA]

Carl - no doubt the resonate frequency will be stimulated and dominate. I'm not sure I agree the "other" data is worthless.

I actually figured it would be much more effective to buy an accelerometer and put it on the platter and hook it to the scope. Leave the arm/cart out of the test. I know I have a suitable device somewhere, just need to find it.

 

[jderimig]

Leave the arm/cart out of the test.

But that is the purest ideal response. No coil (or magnet) movement from the outside the intended excitation source. Smaller is better. Once you find the resonant frequency experimentally hang weighted balls on spring on the base until the amplitude is minimized. Then add damping to reduce it further. Sounds like a fun project.

 

[FredA]

Totally agree that providing energy that stimulates the resonance is bad and something to be avoided. I'm hoping to eliminate all noise sources including those that hit the resonance and those that do not.

Butcher-block came today.
Put it in place and got to listen to one song.
Bass sounds better.
More to come