Impedance matching problem

Let me start by saying that I am NOT an expert. I am building a step attenuator to run a passive preamp for my VTA high gain ST70. I have read quite a bit about choosing resistors and matching the output impedance of the preamp to the input impedance of the amp. The most common recommendation that I have found is a ratio of 10 to 1. Since my amp has an input impedance of 270k ohms, I am selecting resistor pairs (ladder T attenuator) to give an output impedance of around 27k ohms. This is fairly easy to do at lower attenuation but at even mid levels of attenuation I am getting really high resistor values, particularly for the input resistor. For example at 25dB I have an input resistor of 500k and the resistor to ground of around 29k. The input resistors start going up dramatically for higher attenuation. Am I worrying too much about matching at the 10 to 1? In fact at 45db the input resistor is 4.9M. Is all that resistance going to muddy the sound? By the way, my source is a CD player with 2v output.

Thanks for any advice. Please let me know if I am way off as I am new to all of this.

yes, all that resistance IS going to muddy the sound, and it'll be hell trying to build something like that.
Bob here at Tubes4HiFi has a ready-made high quality stepped attenuator that fits right into the front panel of an ST70, for only $50.
It's about 1/4 of the way down this page: www.tubes4hifi.com/bob.htm

Thanks for your suggestion. I looked at Bob's attenuator and decided I would like to build my own. First I wanted the ability to adjust the balance so I will ultimately build two attenuators, one for each channel. Second, I liked the flexibility of picking my own resistor values to best match my setup.

I'm further down the road on this project, as I realized that by worrying too much about matching the impedance ratio from preamp to amp I was ignoring the ratio from CD player to preamp. I changed my resistor selection strategy to basically a compromise where I kept those two ratios in line with each other. Rather than 10:1 I now run anywhere from 35:1 to 50:1. That led to much more reasonable resistor values, 100k & under for the input resistor and 33k & under for the ground resistor. To test it I am going to select about 4 or 5 steps, solder it up and see how it sounds.

If anyone has any comments or suggestions, I would appreciate your help.

Your resistor values are all wrong. The 2 resistors for each step should equal about 27K. I would suggest using 25K. Here are the values.

http://www.hificollective.co.uk/pdf/ladder_r_values1.pdf

If you are using more steps you should be able to figure out the extra resistor between the ones on the chart.
I would not build one, I have done it in the past, but now you can buy a steeped 25K from china with 1% dale resistors for about $50 plus shipping.

You will need to explain this to me. Doesn't adding the resistor values give you the input impedance of the attenuator? Isn't the output impedance calculated like parallel resistors? If so, the 25k attenuator you've shown will produce an output impedance of only 1.2k ohms at the middle step, where the vta st70 is expecting about 27k?

are you trying to build a series attenuator or a ladder attenuator? big difference
there are LOTS of web sites with TONS of info on how to do it and the websites will automatically calculate the resistors for you.
when in doubt, use google !

I am building a ladder type attenuator. I did start with some online resistor calculators. Each seemed to come at the problem from different angles so I ultimately went back to the basics of voltage ratios and impedance ratios. I built up a test attenuator and tried it in my amp and was surprised at the results. The first conclusion was that the VTA based ST70 is incredibly responsive. I started with what I thought would be a lot of attenuation and instead of barely any volume I got a full, rich, albeit quiet sound. Not that that's a bad thing but I didn't expect it. I ended up increasing the input resistance to get it to a point that I can dare listening to after my wife and kids have gone to sleep.

On the other end of the volume spectrum, it is too loud to listen to but only on the edge of distortion. Right now my attenuator consists of 6 steps, but it is already a huge improvement over a shunt-to-ground pot.

Let me give you an example. My preamp has an output impedance of only 200 ohms and it sounds great hooked up to a Dynaco 70. Actually they both sound great. The 10 to 1 is a minimum ratio My preamp would be 135 to 1 and the Dynaco loved it.
If you wanted to test something I would test the low end of the spectrum like a 27K resistor with a 20 ohm resistor. That would give you about 54db of attenuation. If that didn't cause you a problem then no combination of 27K above that would be a problem. You could then keep lowering the value of that 20ohm say 10 ohm then 5 ohm until you had a problem with the amp. Or even easier, Hook up a 25K audio pot. and see if there is a problem. I don't think there will be. Good luck

Thanks for the advice. I will give it a try. Your earlier post gave me an idea that I might try out. The link you gave showed several attenuators that used different resistor pairs to get to the same dB of attenuation. My idea is to put 3 or 4 of them together all at the same level of attenuation but different pairs and listen to the difference. I am guessing that your suggestion for a 25k will be best at the higher end of the range and 50k or even 100k may sound better at the lower volume levels. I did notice that the tables you linked to change the input resistors at the higher volume levels, presumably to attempt to match impedance.

Thanks for the advice and the ideas. It's greatly appreciated.

using a good quality 25K or 50K pot will also allow you to disregard all the overload of info on choosing resistors.
You turn the pot, measure it from both ends to center, and you've got your resistor values at each setting you want. Simple, foolproof.

Ok I finally got around to building a pair of step attenuators. Since I wasn't sure if it really was going to be a significant improvement over a standard pot I just used 1/4 watt carbon film resistors. Well I am really surprised at how much better it sounds. It really cleaned up the mids and highs. I swear even the bass is a bit crisper. It is markedly better. The other pleasant surprise is how much better it sounds at low volumes. Even at low levels the sound still seems to jump out of the speakers and fill the room.

I just placed an order for vishay-dale's to build what I hope is an even better sounding volume control. If anyone is interested in the resistor values I ended up using just let me know.

Report back on this. Photos too.

FYI - I run a passive preamp. A DACT 100K attenuator with the BENT Audio remote kit is at the heart of the passive pre. It works great and sounds excellent. I had Bobs 100 K attenuator and was real happy. I wanted a remote control as I'm lazzy and wanted the equipment all in 1 spot.

I did have a problem with my digital comcast cable box.. It has less gain and it is really noticable compared to my CD player and FM tuner. I got a DECWARE ZSTAGE. The ZSTAGE is a simple gain stage. Worked magic.. Just a single 12AU7 or 12AT7 tube that works sort of like a buffer with an attenuator for gain if desired. Bumps up a signal up to 4 volts or so. Sort of a acitve pre but stripped and pure.

Nice detail builing your own attenuator. I don't know if many do it now.

You have hit the heart of the problem and the reason I went through all the effort. Trying to match, or at least attempt to balance the impedance is the key. Sounds like you have found a way to get there. This whole project was my attempt at testing whether or not it made a significant difference. The answer is yes, it does. The DACT with a remote is a significant upgrade. Is there a DIY version of the DACT? If there is that may be the ultimate solution.

I have the same issue with input sources. Not that I would use my iPhone for serious listening, but it would be nice to at least try to adjust the input voltage to get to a reasonable match. I will need to dig into your gain stage solution. If I could get the output up to the right level that could be the answer.

I will post photos when the whole thing is put together. It is literally a breadboard solution at the moment.