Audio Modifications
While the building blocks that comprise the C and CX versions of this preamp are the same, the stuffing in between those blocks is different -- not to a huge degree, but different none the less. I'll address the phono preamp differences first, as their differences are rather simple, with the remainder of the information pertaining to the line stages. But first, an overview of the basic differences will help.
Besides the rectifier tube, the C uses 6 audio tubes to the CX's 9. The differences here lies in the fluff used to create the additional features of the CX over the more basic C version. Both units use virtually the same phono preamp circuit, with the C then passing the line level signal through 4 stages, while the CX passes them through 5 before appearing at the main output jacks. The 3 extra tubes in the CX support the rather unique Stereo Dimension control, the blended Center and Remote Control outputs, and cathode follower output stages for the main output jacks of the CX unit. Again, this is all fluff, as between the main building blocks of these two units, all the necessary amplification is available to achieve the necessary gain with wide response, and a low output impedance. Of note however, the line stages of the C version provide a measured gain of 8.7, while those of the CX version provide a measured gain of 10.9. This difference is ultimately of little significance and unchanged with the modifications suggested. Also, BOTH of these units invert absolute polarity, which is important to know if this matter concerns you.
PHONO PREAMPS --
Within the phono preamps, the only changes necessary to bring them up to CX status are to:
1. Add 100K shunt resistors at the RIAA 2 input jacks, and in the earliest versions of the C, add them to the RIAA 1 input jacks as well, if not already present. This will cause both of these inputs to reflect a more standard 50K input impedance for magnetic cartridge applications.
2. Within the preamp module, the 100K resistor currently connected to pin# 2 of the preamp tubes should be moved to the other side of the 10K resistor also connected to this terminal. This move allows the 10K resistor to still effectively act as a grid stopper resistor, but eliminates the voltage divider effect these two resistors created in the original design. By preventing a near 10% signal loss then, this move improves the S/N ratio of the unit by producing 10% more output from the phono section, without using any additional amplification to achieve it.
3. Ensure that the interstage coupling cap is effectively a value of .01 uF -- either by way of a .01 uF cap, or two .005 uF caps connected in parallel like my unit had. Yes, the CX used a .022 uF in this position, but it also used a 4.7 meg following stage grid return resistor, where as the C uses a 10 meg resistor in that position. Therefore, the time constant is the same when the C uses only a .01 uF cap for inter-stage coupling.
The phono preamp output stage coupling cap in the C's phono section is also only a .022 uF versus the CX's .047 uF. But again, that's because the preamps in the CX are each driving TWO 250K output level controls, where as in the C, they are only driving one. So again, the time constant is the same in the C with the smaller .022 uF cap in this position.
It is because the CX's phono preamps are driving two level controls that the output stage grid return resistor was lowered, and the two coupling cap values were increased. In spite of these changes however, the response of the phono circuits between the C and CX/2 versions of this preamp is identical.
LINE AMP STAGES --
The line amp stages of the 400C were designed at time when phono was king, and FM was up and coming. It was also designed to be the central figure of THE home entertainment system under all conditions of use. This produced two results:
1. The LF response was appropriate for the available mediums of the day, while midrange response was slightly enhanced, and
2. The volume and loudness functions were very accommodating for low volume listening conditions after little ones went to bed.
Taking the volume/loudness functions first, the design of the loudness circuit in the 400C makes for greatly reduced volume control action in the first half of rotation, throwing most of the control's action beyond the 1:00 setting. This does accommodate low level listening very nicely, but today, it can also throw more normal listening levels outside the control's range where the loudness function operates for those who wish to use it. After all, our hobby is such that when this equipment is on today, it's usually because we are actively LISTENING to it! Additionally, the C's loudness circuits do not provide HF enhancement to go along with the two step LF enhancement provided by its loudness circuit, resulting in a somewhat muddy result from the use of this control.
Fisher realized this heavy skew towards low level listening left a weak impression against the increasingly dynamic music and mediums of the 60's, and so made a change to the loudness circuit that was introduced into all of the premium product lines following the 400C, since all of those units used the same volume/tone control design of this unit. The result is that the volume control action was enhanced in the first half of the control's rotation, the loudness function was changed to a simpler one step circuit, and HF compensation was added into the loudness action as well. It works beautifully. Since the volume control is the same between the C and CX units, all that is required is to change the loudness control components associated with it to those used in the CX/2 models. In my implementation of this change, the first loudness switch position provides both the low and high frequency boost of the CX model, while the second position provides just the LF boost of the CX model, without the HF boost added. That makes this setting similar to the characteristics of the original C model.
Even if you do not use the loudness function regularly, this is still a very worthwhile modification to make, as the increased volume control action below 12:00 is a welcomed change.
Regarding the response issues, today, digital mediums have extended deep bass information, going well below that which was available at the time the C was being produced. That, plus the fact that electrically larger components of the day that would accommodate an improved LF response could not physically fit into the space provided, so the LF response ended up being reduced by today's standards. Also, with the response accuracy of today's mediums, there is no reason to artificially boost any portion of the audio bandwidth when set for flat response. These issues are easily corrected by simple component swap outs as follows. Using the Sam's schematic for the left channel then:
1. C22 should be changed to .005 uF. This flattens the response of the stage, eliminating the slight mid frequency boost.
2. C23 should be changed to 1 uF (non-electrolytic).
3. C4 should be changed to a 47 uF.
4. C28 should be shorted out and eliminated.
5. C5 should be changed to 100 uF.
6. C31 should be changed to 1 uF (non-electrolytic).
Make the appropriate changes in Channel B as well. Additionally, I found that C50 in my unit was more accurate at 10 pF, rather than with the 8 pF supplied, although such a change will not be audible. It's mate C29 was found to be accurate as is.
These changes will allow the 400C to have all the LF response of the CX/2 versions, and the same flat response characteristic throughout the audio bandwidth as well.
The results of these changes collectively make for a very much improved 400C. The volume control is responsive and well positioned now for power amplifiers of typical sensitivity. The full bass register is there with authority, without any false emphasis throughout the entire audio spectrum. The loudness function works superbly, and together with the changes made to eliminate hum, it is now dead quiet now as well -- even with the improved LF response it now sports.
Fisher must have thought these were worthwhile goals as well, as they were all improvements added into their CX/2 models as well. In all my efforts to modify/improve/enhance Fisher equipment, I never try to turn them into something they are not, or cause them to make a spectacle of themselves. Fisher equipment is typically known to sit quietly ready, rising to any challenge given it, and accomplishing that task with unequaled poise, character, and class. The modified 400C accomplishes all these things qiote ably now. I would like to think that Avery would approve.
In the last post, I'll detail some worthwhile power supply changes to make the unit a little more forgiving to its tubes, and take care of a few small socially unacceptable behavioral issues. I'll also post a few scope shots of the final response achieved.
Dave