ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
classic: The capacitor C3 = 4.7nF in the RIAA circuit (it looks like during the different drawing versions I have made an error and used the value 47nF) I am really sorry for all your work by finding the reason for the problems of getting 40dB gain. I found out the error by going all the way back to my simulation circuit and compared with the circuit in the PDF file. In my own RIAA I have used 4.7nF that explains why I get the correct measurements. /Frede
The capacitor C3 = 4.7nF in the RIAA circuit (it looks like during the different drawing versions I have made an error and used the value 47nF)
I am really sorry for all your work by finding the reason for the problems of getting 40dB gain.
I found out the error by going all the way back to my simulation circuit and compared with the circuit in the PDF file.
In my own RIAA I have used 4.7nF that explains why I get the correct measurements.
/Frede
No problem. It is good to have the answer and confirm that the math doesn't lie :). With both cards behaving the same it had to be something like a difference in parts. I ordered some 4.7nF caps today.
The 4.7nF caps arrived today. Here is a picture of them installed.
The new caps did the trick as Frede said they would. With a 3mV input signal I can now adjust the RIAA amp outputs to a 40dB gain. Right now my inputs are a little out of balance. I need to recalibrate my E-MU sound device. The left channel input is about 3.06mV and the right channel is 3.15mV. So I adjusted the trimmers on the left and right RIAA amp boards for the respective outputs shown on the screen. With the iPod 3mV output directly to the E-MU device, I am seeing a solid sine wave. On the Audio Tester, with power on but no input applied, I am seeing some noise on the RIAA amplifier output. I have tried different ground jumper settings but I am wondering if it is a problem with the wires (although short) from my DIN plug to the RIAA board. I will have to look at that next. Here are the RIAA outputs for the left and right channels. You can see there is a slight wave on the signal. The right channel is the red trace. Left channel is the blue trace.
Continuing on...I have tried various tests involving the Audio Tester and the PC software measurements (using the E-MU sound device) as well as an HP8903A audio analyzer. I think it is time to try measuring an actual piece of audio equipment. I have chosen a Beomaster 5500. It is in working condition. I bought it last year but haven't spent any time with it other than to verify it works. No work has been done on it to my knowledge.
Looking at the BM5500, I decided to use the Tape 2/Aux (TP2 AUX) input as the point of applying a test signal and the Line out as the point for measuring the output signal. There are L & R jacks labeled PRE AMP but I can't measure a signal out of them so I'm not sure what their function it. On the BM5500 schematic it looks like those lines are tied to power amp outputs so I am confused by the PRE AMP label.
I connected the BM5500 TP2 AUX inputs to the Classic Audio Tester outputs and the BM5500 LINE outputs to the Classic Audio Tester inputs. I decided to start with just some inputs and outputs in the pre-amp section (on output amplifier testing yet) so I don't have to worry about the voltages going to the Classic Audio Tester. The audio tester inputs must be below 1.8 volts. I do have one fixed power resistor load I can use later on the speaker outputs. It is an 8 ohm resistor with a large aluminum heat sink that will handle around 250 W. For now, the next step is to power up the audio tester and run some tests with the Audiotester software. The pictures here show the test setup.
Here is the spectrum analyzer measurement of a 1k Hz signal sine wave. The test was made using Audiotester v3.0 software to apply the test signal and measure it (through the E-MU 0204 and the Classic Audio Tester to the BM5500). I ran the same test varying the signal frequency from 20 Hz to 20,000 Hz. I found that this test wouldn't work well below 50 Hz. From 50 Hz to 20,000 Hz though, I got pretty much the same results.
The next test I ran was the set of audio tests by the Rightmark v6 software. The first picture shows the pre-test check of the software to verify the input levels. The second picture is of the test results. The tests are different than the single frequency sine wave tests I ran with the Audiotester v3 software so the results are not the same. They look pretty close though. What did these tests really check on the BM5500? The Tape 2/Aux input signal goes through IC1 (a program select latch), then through an op-amp (IC2 - LF 353) and out the Line outputs. So to test the effects of the tone controls and the power amplifier I will have to setup my load resistor and carefully plan a test at low volume.
I connected some S55 speakers to the BM5500 speaker 2 sockets and verified my 1k Hz signal was at the output. My oscilloscope isn't picking up anything on the BM5500 PRE AMP output (RCA) jacks though. In reading some posts on the archived Beoworld forum I see that the topic of the BM5500 PRE AMP outputs has been discussed already. They are wire to the power amplifier output instead of between the pre-amp stage and power amp stage. They are also attenuated (divided by 19). However, I am not seeing any signal there. Maybe someone disconnected those internally?
It has been a while since any updates have been posted to this thread but Frede's Classic Audio Tester is still on my workbench and lately I started playing around with audio measurements again. Since I built Frede's tester kit back in 2013 I have bought a QuantAsylum QA400 Audio Analyzer. It can be used with Frede's tester as an option to using the E-MU 0204 Audio device or the QA400 can be connected directly to some audio signal to measure its signal.A nice feature of Frede's test interface though is that it has a built in RIAA amplifier for testing turntables. The problem for me with that part however was that I couldn't get rid of the 60Hz hum. This evening I decided to go back into the tester and see if I could find and fix the problem. I am happy to say I finally fixed it. The problem is as you would guess...a grounding issue. The tester allows for a Beogram DIN plug to connect to a DIN jack on the front panel. The phono signals (L/R) go through the RIAA preamplifier boards and can be routed out of the rear tester RCA jacks to a computer for measuring. The RIAA outputs also route out to a couple of RCA jacks on the front panel of the tester where they can easily be connected to an amplifier.My grounding problems were two-fold. First the DIN jack case shield pin needed to be connected to the tester box chassis ground. Second, my front RCA jacks did not have their shields wired correctly. I had them going to the tester chassis ground when they should have gone to the RIAA L/R board return.Once I corrected those two problems my RIAA preamp worked like a charm. There is no audible 60Hz hum from the front or rear RCA jacks. Here is a picture of the corrected wiring.