New Older S.E.X. kit completed problems.

[bliss53]

Greetings,

I am new to the forum.  I just completed a S.E.X. kit.  It was purchased for me as a gift last April.   It is my first build and I enjoyed it very much.  I have a couple of problems I am hoping someone can help me with.  I tried the amp on some throw away speakers and they worked so I tried with some grado 225 headphones, shure 840 headphones and klipsch cornwall speakers.  I found that the sound was good and the background quiet but the volume was pretty low on all the headphones and speakers even at maximum volume.

I do have the following unusual values:

Resistance:

14,34 = Both are a constant reading of .249
A1,B1 = Both are a constant reading of .249

Voltage:

A7,B7 = 10.9/7.7
C4=-4.0
Terminal 7 = No constant VAC reading, VDC reads 182
Terminal 10= No constant VAC reading, VDC reads 184

I have gone over the instructions and checked all the solders.  I know I am missing something.  Can anyone help me narrow it down from the situation and values above?

Thank you,

Peter

[Paul Joppa]

It seems most likely that you are not noticing the meter scale.

For resistance, 14, 34, A1, and B1 should read 249000 ohms, which is the same as 249K ohms or 0.249M ohms.

For voltage, A7, B7, and C4 are connected together and to chassis ground at C1. There may be tiny voltages between the chassis ground at the back of the amp, and the signal ground used fro voltage measurements. It is likely you are measuring 4.0, 10.9, and 7.7mV, which are the same as 0.004, 0.0109, and 0.0077 volts - in other words, nearly zero volts.

If the loudness is badly reduced, it may be a quiet source or a problem with the volume control wiring. What is the source?

[bliss53]

The source is a DAC connected to computer with itunes and Pure Music.  Pure Music allows a change in the output signal.  I increased the output a bit but any more than a few DB added distortion.  I will get an rca to ipod cable tomorrow to try a different source.

[bliss53]

I forgot to thank you for the reply.

[Paul Joppa]

Most DACs will put out plenty of voltage; it takes less than one volt to drive the SEX amp into clipping. I don't see any reason a music server would allow itself or the attached DAC to overload, so that distortion seems suspicious. By all means try another source if it's easy to do, just in order to confirm that the source is not at fault. But it seems unlikely, especially if the source has been working into another amp.

The Grados and Cornwally are pretty efficient, and you don't indicate that you like your music at deafening levels, or are nearly deaf yourself. So it's not likely that you are suffering form the admittedly low power of the amp.

I just mention these things for completeness; we are diagnosing a problem by testing a system so it's always a good idea to be sure we are looking at the right component.

So ... back to basics. Are all the resistance and voltage measurements correct, as described in the manual? Again, I expect you did these tests but wanted to check for sure. Plus, you should probably test again to see if my guess about the measurement problems in the original post was correct or not. Also, are both channels showing the same symptoms? Is the sound balanced, i.e. both bass and treble are present in reasonable proportion to each other? We need more clues to solve this puzzle!

[bliss53]

I have used this dac set up with conrad johnson (PV5,MV75) and mcintosh (C11, MC75s) gear with no problems.  With the cornwalls I never go above about 1/3 on the volume dials on both sets of gear and that will wake the neighborhood.  As far as headphone volume I have a little portable FIIO iphone amp that puts out 120-300mV at 16 ohms and it can get well above my volume comfort level.  The S.E.X. amp is still well within my comfort level at maximum.

The sound I got was good well balanced across the spectrum and the image was centered.

I reviewed the volume controls wiring and I did not find any errors.

I will go back and retest all the values this morning and identify "mV" as it appears.  I will also try to post some pictures.

Thank you again for your assistance.

[Grainger49]

Bliss53,

There are just too many functioning SEX amps out there with every conceivable input that don't have this problem.  I'm likely to say that something is awry.  

Back in the dark ages when I was selling audio gear, I suspect you were around then, there was a tendency to blame whoever had it last.  I heard this quote, more than once, "these speakers have hiss in them."  In this case it can be whatever component was added most recently.  I'm equally to blame at this myself.

What I'm saying is just hang in there and everything will be solved.  Trust me (why would you?  Maybe because I've been there and I suspect so have you).  

[Doc B.]

Do you have the 10 ohm wirewound resistors across the outputs? If so, get rid of them, they were eliminated in later kits. You can also replace the 120 ohm resistors at the headphone jack with straight wire. You should be able to blow your eardrums out your nose with the 2 watts of the SEX amp running through most headphones.

[Paul Joppa]

Just a short note here, on volume controls. They do not control the output level per se, they just control the gain. Since most amps will produce their full output with something on the order of 1 volt input, higher power amps will naturally have a higher maximum gain, and so the gain control on a high power amp will be set to a lower point compared to a low power amp which is producing the same actual power into the speaker.

The stock SEX amp has a maximum gain of about 6, which is 16dB.

Once you have done the resistance and voltage measurements, if that does not identify a problem, it is likely time to make some measurements of the levels and gains. Fortunately you have a digital front end and can download MP3 files of various signals, so this should be relatively easy. Here's one of the first I found with google: http://www.stecrecords.com/?RecordId=581

[bliss53]

I tried an iphone as a source and it performed the same as the DAC.  There was a little distortion at maximum amp volume and maximum iphone volume (headphone out to rca plugs).  My meter is an auto so it is a little confusing.

Here are all the values:

Resistance in ohms:

1,21 - both non zero
2,22 - both non zero
4,24 - both non zero
5,25 - 0,0
6,26 - both non zero
9,29 - 0.610k,0.607k
10,30 - 0,0
11,31 - both non zero
12,32 - 0.610k,0.608k
14,34 - 0.249m,0.249m both not fluctuating
15,35 - 0,0
16,36 - both non zero
17,37 - 0,0
18,38 - 0,0
19,39 - 1.3 min/98.2k max, 1.5 min/101.2k max
20,40 - 285k,295k
A1,B1 - 0.249m/0.249m both not fluctuating
A2,B2 - both non zero
A3,B3 - 0,0
A4,B4 - 0.229k min/98.4k max, 0.244k min/101.2k max
A5,B5 - both non zero
A6,B6 - 1.27k, 1.27k
A7,B7 - 0,0
A8,B8 - 0.4,0.3
C1,C2,C3,C4 - 0,0.3,0,0

Voltage

1,21 - 185,187
2,22 - 185,187
4,24 - 183,186
5,25 - 0.3mv, 0.1mv
6,26 - 385,389
9,29 - -17,-16
10,30 - 0,0
11,31 - 367,371
12,32 - -17,-16
14,34 - -17,-16
15,35 - 0,0
16,36 - 350,354
17,37 - 0,0
18,38 - 0,0
19,39 - 0,0
20,40 - 0,0
A1,B1 - -17,-16
A2,B2 - 344,349
A3,B3 - 0,0
A4,B4 - 0,0
A5,B5 - 64,72
A6,B6 - 2.4,2.4
A7,B7 - 10mv,8mv
A8,B8 - 5.8,5.9
C1 - 0
C2 - 5.9
C3 - -4.0mv
C4 - 6.0

Terminal 1 - 117vac
Terminal 2 - 1mvac
Terminal 4 - 2.9vac
Terminal 5 - 2.9vac
Terminal 6 - 159vac
Terminal 7 - fluctuating vac reading, 183 vdc
Terminal 9 - 160vac
Terminal 10 - fluctuating vac reading, 184 vdc

Choke Terminal 1,8 - 353/353, 358/358 vdc
Choke Terminal 4 - 363,368 vdc
Choke Terminal 5 - 343,349 vdc

I did not add the 10ohm resistors across the outputs.  I will leave the stock 120 ohm resisters in place until I fix the stock set up.

Thanks for the encouragement.  This is my first time with this sort of thing so I know I screwed up somewhere.

[Paul Joppa]

T20, T40 should be 295 ohms (0.295K). You reported 285K, 295K. I assume that is a reporting error?

Other than that, the numbers all look very good. So I expect that any problem will be in the input or output connection. So let's trace the signal voltages. I'll suggest using a sine wave signal because it is steady in amplitude and easily measured. You can download mp3 files from many sources, for example the one I mentioned before has a 440Hz tone which is very suitable. Download the file, play it (set to auto-repeat if you can), and measure the AC voltage output of the DAC; using your software set it for 100mV output signal level.

Now we'll be connecting this to the SEX amp, and a pure tone can be pretty annoying after the first few minutes, so you might want to use those 10 ohm resistors in place of the speakers as a load. I want to evaluate the speaker outputs first, so no headphones yet. We'll be measuring signal voltages inside the amp, so re-read the hints on making safe measurements and be very careful! I'll describe one channel, just add 20 to the terminal numbers for the other channel.

With the amp on, measure the AC voltage at T19. It should vary from zero to 100mV as the volume control is adjusted. This confirms the signal is getting to the first triode in the tube.

The next measurement is the output of the driver triode. This signal is biased by the DC voltage, which is +70v at one end of the coupling capacitor and -17v at the other end. Many meters will not accurately measure the AC component of a voltage  when there is also a DC component, so we much change the "ground" reference for this measurement.  There is a 249K resistor from T12 to T14; measure the AC voltage across this resistor. It should be about 2.0 volts if the volume is turned up all the way. This indicates that the driver is amplifying the signal.

Next we'll measure the output of the power triode. This is referred to signal ground again, and the purpose of the large parafeed cap is to remove the DC component. So look for the AC voltage at T20. It should be about 20 volts, indicating the power triode is further amplifying the signal.

Finally, we'll measure the output. The output transformer reduced the voltage by a factor of 30, so look for about 0.6 volts at the output terminals (speaker terminals).

Hopefully the signal will be lost somewhere along this line. If it is not and all these tests are passed, increase the signal from the DAC to 0.5 volts and repeat, looking for 10v, 100v, and 3v along the way.

[bliss53]

Thanks again for the detailed response.  Finally got some time to do the testing.

You are right I forgot the decimal point on the T20 and T40 ohm measurements

The output signal from my dac with the software scale at 0db was .213v and .209v.

At the outputs levels above I got the following test values

T19/T29 = min .001v/.000v, max .215v/211v
T12/T14 = 3.14v
T32/T34 = 3.25v
T20/T40 = 31.6v/32.9v
Speaker Terminals R/L = 1.09v/1.0v

I am not sure where to go from here.  I turned my software up to its max at +12db.  The output signal at this level was .431v/.423v.

I wish I knew more about this stuff to make it easier on you.  I guess everyone has to start somewhere.

See the pictures attached.

[Paul Joppa]

OK, it appears that the amp is working correctly, but your dac and/or software will not produce a large enough signal to drive the SEX amp to full power. That's a disappointment for sure!

If you get 0.21 volts at 0dB, then you should get 0.84 volts at +12dB. But you say you are getting half that. That may point to a problem in the DAC and/or software - I'm not an expert on that.

Fortunately, there are some digital experts around on this forum. It would be a good idea to identify the DAC, and its connection, in the hopes that someone is familiar with it. You already said you are using iTunes; are you on a Mac or a PC?

If this doesn't produce any help in a day or two, you might start a new thread since we are well down the page on this one and the title does not represent what I think the problem may be at this point.

[Grainger49]

You might want to post the DAC problem in the Digital folder.  Fewer people will see it here in the SEX folder.

[2wo]

Can you connect to a CD or DVD player?...John