dmp
Well-known member
This week I'm racking up a pair of Telefunken v672 preamps and just got them up and running.
After listening and analyzing them with RMAA, I found they have some high frequency rolloff that became very significantly at high gain. Since I found this issue has been asked about a few times on the forum, I thought I'd post the results from my analysis and solution. In summary, I found that the high frequency rolloff was not intrinsic to the preamps and could be eliminated with proper adjustment and wiring.
First, at 30 dB (moderate) gain (which I believe is appx. what these amps were designed to be fixed at)
I found one of the v672s had a rolloff dropping about 2dB from 10kHz to 20 kHz:
I found a schematic that was pretty close to my version of v672s, shown here for reference. I believe my v672s are early versions that had dual output transformers and all transistor signal path (no ICs).
I found that my v672s had a trimmer (R210) in combination with a resistor instead of the single R201, shown on the schematic. This resistance apparently sets a rollover frequency for a low pass filter in combination with C202. I think this is referred to as the broadcast filter in manuals. By turning the trimmer, this frequency can be moved above the audible range. In the schematic, the 4.7k resistance would translate to a 22kHz 3dB rollover. My v672s have a 3.9k resistor in series with a 3k trimmer, allowing the 3dB point to vary from 15kHz to 27 kHz. I found one of my v672s had the trimmer adjusted for a lower rolloff frequency. After adjusting the trimmer, I found both v672s now showed no rolloff below 20 kHz (adjustment of the trimmer could probably pull down the slight bump at 20 kHz if desired).
Next, I looked at the frequency response of the v672s at 54 dB (high) gain (which is set with an externally connected resistance) and found a much more significant high frequency rolloff. This graph shows the high gain frequency response (54dB), in white, compared to the 30dB moderate gain response, in green.
Looking at the schematic, the V672 gain is set with negative feedback from between the two amp stages to one of the windings of the input transformer, adjusted with an externally connected resistor (Rg) at pins 9 & 10. The external gain resistor is in parallel with C1, a 15 pF capacitor. C1 lets some the high frequencies past, creating a HF rolloff. Depending the on the value of Rg, the rollover frequency for the RC pair may be in the audible range. For lower Rg (like the setting for 30 dB gain) the frequency is above 20 kHz, but with larger Rg, for high gain (Rg ~1meg), the rollover frequency lowers into the audible range. The rollover frequency for an RC pair is the standard formula, 1/(2piRC).
I removed C1 from one of the v672s (shown in green) and found that the high freq rolloff, although still present, was not as significant.
Because very small capacitance clearly affects the gain of the circuit at high Rg, the capacitance of the twisted-pair hookup cable I was using to connect the Rg pot could have an impact. I used Belden 1508A cable, which, after checking the datasheet, has 31 pF/ft. Since I was using about 1ft to connect to the frontpanel of my case, this capacitance was significant. To test this, I removed the hookup cable and connected the Rg pot as close as possible to the preamp. Looking at this with RMAA, the v672 with the Rg pot moved and C1 removed, shown in green, no longer has high frequency rolloff.
So, in summary, the broadcast low pass filter can be adjusted out of the audible range by with the trimmer (R210). High frequency rolloff at high gains can be caused by poor wiring that introduces parallel capacitance and C1, a capacitor in parallel with the external gain resistance. Removing C1, and using separate shielded wires to connect Rg, should prevent the high freq rolloff I observed.
After listening and analyzing them with RMAA, I found they have some high frequency rolloff that became very significantly at high gain. Since I found this issue has been asked about a few times on the forum, I thought I'd post the results from my analysis and solution. In summary, I found that the high frequency rolloff was not intrinsic to the preamps and could be eliminated with proper adjustment and wiring.
First, at 30 dB (moderate) gain (which I believe is appx. what these amps were designed to be fixed at)
I found one of the v672s had a rolloff dropping about 2dB from 10kHz to 20 kHz:
I found a schematic that was pretty close to my version of v672s, shown here for reference. I believe my v672s are early versions that had dual output transformers and all transistor signal path (no ICs).
I found that my v672s had a trimmer (R210) in combination with a resistor instead of the single R201, shown on the schematic. This resistance apparently sets a rollover frequency for a low pass filter in combination with C202. I think this is referred to as the broadcast filter in manuals. By turning the trimmer, this frequency can be moved above the audible range. In the schematic, the 4.7k resistance would translate to a 22kHz 3dB rollover. My v672s have a 3.9k resistor in series with a 3k trimmer, allowing the 3dB point to vary from 15kHz to 27 kHz. I found one of my v672s had the trimmer adjusted for a lower rolloff frequency. After adjusting the trimmer, I found both v672s now showed no rolloff below 20 kHz (adjustment of the trimmer could probably pull down the slight bump at 20 kHz if desired).
Next, I looked at the frequency response of the v672s at 54 dB (high) gain (which is set with an externally connected resistance) and found a much more significant high frequency rolloff. This graph shows the high gain frequency response (54dB), in white, compared to the 30dB moderate gain response, in green.
Looking at the schematic, the V672 gain is set with negative feedback from between the two amp stages to one of the windings of the input transformer, adjusted with an externally connected resistor (Rg) at pins 9 & 10. The external gain resistor is in parallel with C1, a 15 pF capacitor. C1 lets some the high frequencies past, creating a HF rolloff. Depending the on the value of Rg, the rollover frequency for the RC pair may be in the audible range. For lower Rg (like the setting for 30 dB gain) the frequency is above 20 kHz, but with larger Rg, for high gain (Rg ~1meg), the rollover frequency lowers into the audible range. The rollover frequency for an RC pair is the standard formula, 1/(2piRC).
I removed C1 from one of the v672s (shown in green) and found that the high freq rolloff, although still present, was not as significant.
Because very small capacitance clearly affects the gain of the circuit at high Rg, the capacitance of the twisted-pair hookup cable I was using to connect the Rg pot could have an impact. I used Belden 1508A cable, which, after checking the datasheet, has 31 pF/ft. Since I was using about 1ft to connect to the frontpanel of my case, this capacitance was significant. To test this, I removed the hookup cable and connected the Rg pot as close as possible to the preamp. Looking at this with RMAA, the v672 with the Rg pot moved and C1 removed, shown in green, no longer has high frequency rolloff.
So, in summary, the broadcast low pass filter can be adjusted out of the audible range by with the trimmer (R210). High frequency rolloff at high gains can be caused by poor wiring that introduces parallel capacitance and C1, a capacitor in parallel with the external gain resistance. Removing C1, and using separate shielded wires to connect Rg, should prevent the high freq rolloff I observed.
